Wide-area lighting fixture with segmented emission

ABSTRACT

A lighting fixture lighting fixture with segmented emission employing a a sheet of an optically transmissive material having a first broad-area surface configured for light output, an opposing second broad-area surface extending parallel to the first broad-area surface and configured for light output, a first edge configured for light input, and an opposing second edge. The lighting fixture further employs a series of compact solid-state light sources optically coupled at least to the first edge, a plurality of patterned light extraction areas distributed over an area of the rectangular sheet and separated from one another and from the first and second edges by separation areas, and a housing encasing the light sources and one or more edges of the rectangular sheet. A width of the separation areas may be less than a length or width dimension of the patterned light extraction areas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/386,472, filed Jul. 27, 2021, which is a continuation of U.S. patentapplication Ser. No. 16/780,872, filed Feb. 3, 2020, which is acontinuation of U.S. patent application Ser. No. 16/125,686, filed Sep.8, 2018. This application also claims priority from U.S. provisionalapplication Ser. No. 62/556,415 filed Sep. 9, 2017, incorporated hereinby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

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NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to wide-area illumination devicesemploying planar light guides and compact solid-state light emittingdevices such as light emitting diodes (LEDs) or laser diodes. Moreparticularly, this invention relates to wide-area LED illuminationdevices such as those employed in lighting panels, lighting luminaires,illuminated panel signs, electronic displays, front lights, backlights,backlit display screens, advertising displays, road signs, decorativebroad-area lights, as well as to a method for redistributing light froma variety of light sources in such devices. The invention furtherrelates to illumination devices in which planar-type light guides areretained in bent or curved state.

2. Description of Background Art

Conventionally, wide-area light emitting devices employ planar lightguides, also commonly referred to as “waveguides”, which are illuminatedfrom one or more edges using Light Emitting Diodes (LEDs) or other typesof compact light sources. The conventional edge-lit illumination systemsmay exhibit certain limitations such as difficulty to efficientlycouple, decouple and/or distribute light. Additionally, configuring theedge-lit illumination systems for a desired angular distribution anduniformity of the emission may be associated with optical losses andlead to energy waste and suboptimal performance.

U.S. Patent Applications Publications No. US-2014-0226361-A1 (the '361Publication) and US-2017-0045666-A1 (the '666 Publication), thedisclosure of which is incorporated herein by reference in its entirety,disclose face-lit waveguide illumination systems formed by a planarwaveguide and optical coupling elements attached to a face of thewaveguide. U.S. Pat. Nos. 9,256,007, 9,097,826, 8,290,318, and U.S.Patent Applications Publication No. US20140140091, the disclosure ofwhich is incorporated herein by reference in its entirety, disclosevarious configurations of waveguides (light guides) and lightdeflecting/light extraction elements.

U.S. patent application Ser. No. 15/996,865 (the '865 Application)published as U.S. Patent Applications Publication No. US20180348423, thedisclosure of which is incorporated herein by reference in its entirety,discloses various configurations of stepped light guides and light guideillumination systems, as well as different arrangements of solid-statelight sources and light extraction features.

U.S. Pat. Nos. D777,972, D776,331, D799,738, D814,101, D824,085,D824,086, and D824,087, the disclosure of which is incorporated hereinby reference in its entirety, disclose exemplary light emitting patternsassociated with light emitting sheet-form structures.

It is noted that, where a definition or use of a term in a reference,which is incorporated by reference herein is inconsistent or contrary tothe definition of that term provided herein, the definition of that termprovided herein applies and the definition of that term in the referencedoes not apply.

BRIEF SUMMARY OF THE INVENTION

Certain aspects of embodiments disclosed herein by way of example aresummarized in this Section. These aspects are not intended to limit thescope of any invention disclosed and/or claimed herein in any way andare presented merely to provide the reader with a brief summary ofcertain forms an invention disclosed and/or claimed herein might take.It should be understood that any invention disclosed and/or claimedherein may encompass a variety of aspects that may not be set forthbelow.

According to one embodiment, a wide-area solid-state illumination deviceor system is exemplified by a thin and flexible sheet of an opticallytransmissive material illuminated by a plurality of compact solid-statesources. The thin and flexible sheet is configured for guiding lightbetween opposing edges using optical transmission and a total internalreflection. At least one broad-area surface defining of the thin andflexible sheet is patterned for light extraction using a plurality ofdiscrete surface microstructures that are distributed over the surfaceaccording to a predetermined two-dimensional pattern. In oneimplementation, at least one of the discrete surface microstructures isformed by a microdrop of a non-absorbing light scattering material thatforms a thin, semi-opaque layer on the respective broad-area surface (a“microdot”). The light scattering material is disposed in opticalcontact with the surface and configured for extracting light from thethin and flexible sheet. In one implementation, the non-absorbing lightscattering material comprises a UV curable ink having light-scatteringparticles suspended in a transparent or translucent binder materialaccording to a predefined concentration. The discrete surfacemicrostructures are so dimensioned and the properties of thenon-absorbing light scattering material are so selected that lightextracted by the microstructures can be output from both opposingbroad-area surfaces of the thin and flexible sheet.

According to one embodiment, a method of making a wide-area solid-stateillumination device, consistent with the present invention, includesproviding a highly transparent sheet of a glass or plastic material,providing a UV printing machine equipped with a recirculation pump and aclosed-path fluid circuit being operable independently from a primaryink supply line, providing a liquid UV-curable white ink having lightscattering particles suspended in a transparent binder material with apredefined concentration and size distribution, printing atwo-dimensional pattern of discrete microdots of the white ink on asurface of the transparent sheet with a variable spacing, curing theprinted microdots to a solid state using a UV light, and at leastintermittently recirculating the white ink within the closed-path fluidcircuit. At least some of the discrete microdots are sized andconfigured to form a semi-opaque light scattering layer.

Various implementations and refinements of the features noted above mayexist in relation to various aspects of the present inventionindividually or in any combination. Further features, aspects andelements of the invention will be brought out in the following portionsof the specification, wherein the detailed description is for thepurpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic perspective view and raytracing of a wide-areasolid-state light guide illumination system, according to at least oneembodiment of the present invention.

FIG. 2 is a schematic view of an adjacent pair of spaced-apart lightextracting features formed on a surface of a light guiding sheet,illustrating exemplary definitions of a spacing distance and aseparation distance, according to at least one embodiment of the presentinvention.

FIG. 3 is a schematic view of contacting light extracting features,according to at least one embodiment of the present invention.

FIG. 4 is a schematic view of substantially overlapping light extractingfeatures, according to at least one embodiment of the present invention.

FIG. 5 is a schematic perspective view of a wide-area light guideillumination system having a segmented light extraction pattern,according to at least one embodiment of the present invention.

FIG. 6 is a schematic plan view of a wide-area light guide illuminationsystem, showing an exemplary alternative arrangement of individual lightextraction areas and separation areas of a light extraction pattern,according to at least one embodiment of the present invention.

FIG. 7 is a schematic view of portion of a wide-area light guideillumination system, showing a two-dimensional array of hexagonal lightextracting sections distributed over an area of a light guiding sheet,according to at least one embodiment of the present invention.

FIG. 8 is a schematic view showing an exemplary variable-densitydistribution pattern of light extraction features for a portion of awide-area light guide illumination system, according to at least oneembodiment of the present invention.

FIG. 9 is a schematic section view of a portion of a wide-area lightguide illumination system, showing an irregularly shaped lightextraction feature deposited to a surface of a light guide, according toat least one embodiment of the present invention.

FIG. 10 is a schematic surface profile illustrating exemplarymeasurements of a surface waviness and a contact angle, according to atleast one embodiment of the present invention.

FIG. 11 is a schematic graph showing a measured dependency of theopacity of a light guiding sheet on spacing between light extractionfeatures, according to at least one embodiment of the present invention.

FIG. 12 is a schematic view of a surface portion of a light guidingsheet having a plurality of light extraction features having variousshapes and sizes, according to at least one embodiment of the presentinvention.

FIG. 13 is a schematic section view and raytracing of a portion of awide-area light guide illumination system, showing light extractionfeatures including at least two layers of different materials, accordingto at least one embodiment of the present invention.

FIG. 14 is a schematic section view and raytracing of a portion of afront light including light blocking areas provided on a transparentsubstrate, according to at least one embodiment of the presentinvention.

FIG. 15 is a schematic section view of a portion of a wide-area lightguide illumination system having multiple stacked light guiding sheets,each including light extraction patterns on both opposing broad-areasurfaces, according to at least one embodiment of the present invention.

FIG. 16 is a schematic section view of a portion of a wide-area lightguide illumination system having multiple stacked light guiding sheets,showing light diffusing sheets positioned between the light guidingsheets, according to at least one embodiment of the present invention.

FIG. 17 is a schematic section view of a portion of a wide-area lightguide illumination system having different types of light extractionfeatures formed in opposing broad-area surfaces of a light guidingsheet, according to at least one embodiment of the present invention.

FIG. 18 is a calculated polar luminous intensity distribution graph foran exemplary configuration of light extraction features, showing asymmetrical Lambertian emission from opposing broad-area surfaces of aplanar light guide with equal top/bottom light output, according to atleast one embodiment of the present invention.

FIG. 19 is a calculated polar luminous intensity distribution graph foran alternative exemplary configuration of light extraction features,showing a Lambertian emission from opposing broad-area surfaces of aplanar light guide with unequal top/bottom light output, according to atleast one embodiment of the present invention.

FIG. 20 is a calculated polar luminous intensity distribution graph fora further alternative exemplary configuration of light extractionfeatures, showing different types of “batwing” emission from opposingtop and bottom broad-area surfaces of a planar light guide, according toat least one embodiment of the present invention.

FIG. 21 is a calculated polar luminous intensity distribution graph fora yet further alternative exemplary configuration of light extractionfeatures, showing a directional emission from a broad-area surface of aplanar light guide, according to at least one embodiment of the presentinvention.

FIG. 22 is a measured polar luminous intensity distribution graph for anexemplary configuration of a wide-area illumination system, showing aquasi-Lambertian emission from opposing top and bottom broad-areasurfaces of a planar light guide, according to at least one embodimentof the present invention.

FIG. 23 is a measured polar luminous intensity distribution graph for anexemplary configuration of a wide-area illumination system, showing anear-Lambertian emission from a single broad-area surface of a planarlight guide, according to at least one embodiment of the presentinvention.

FIG. 24 is a photograph of an ordered pattern of semi-opaque lightextraction features printed on a surface of a light guiding sheet,according to at least one embodiment of the present invention.

FIG. 25 is a photograph of a random pattern of semi-opaque lightextraction features printed on a surface of a light guiding sheet with arelatively high areal density, according to at least one embodiment ofthe present invention.

FIG. 26 is a photograph of an ordered pattern of optically clear,elongated light extraction features printed on a surface of a lightguiding sheet, according to at least one embodiment of the presentinvention.

FIG. 27 is a photograph of a lower-density ordered pattern of opticallyclear, elongated light extraction features printed on a surface of alight guiding sheet, according to at least one embodiment of the presentinvention.

FIG. 28 is a microphotograph of an individual semi-opaque lightextraction feature printed on a surface of a light guiding sheet andhaving a round or near-round shape at the base, according to at leastone embodiment of the present invention.

FIG. 29 is a microphotograph of an individual semi-opaque lightextraction feature printed on a surface of a light guiding sheet andhaving an irregular elongated shape at the base, according to at leastone embodiment of the present invention.

FIG. 30 is a series of photographs of portions of a surface of a planarlight guide, showing different patterns and distribution densities ofmicrodots of a light scattering material, according to at least oneembodiment of the present invention.

FIG. 31 is a three-dimensional composite image of an exemplaryindividual microdot of a light scattering material on a surface of aplanar light guide, according to at least one embodiment of the presentinvention.

FIG. 32 is a graph showing a measured cross sectional height profile ofthe microdot depicted in FIG. 31.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposes,the present invention is embodied in the systems generally shown in thepreceding figures. It will be appreciated that the systems may vary asto configuration and as to details of the parts without departing fromthe basic concepts as disclosed herein. Furthermore, elementsrepresented in one embodiment as taught herein are applicable withoutlimitation to other embodiments taught herein, and in combination withthose embodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection and distribution of electromagnetic radiant energy,such as light, in a broad spectrum or any suitable spectral bands ordomains. Therefore, for the sake of simplicity of expression, withoutlimiting generality of this invention, the term “light” will be usedherein although the general terms “electromagnetic energy”,“electromagnetic radiation”, “radiant energy” or exemplary terms like“visible light”, “infrared light”, or “ultraviolet light” would also beappropriate.

Furthermore, many applications exist for the present invention inrelation to distributing light by means of a planar optical light guidewhich hereinafter may also be referenced to as a waveguide. The planaroptical light guide (waveguide) refers to a broad class of objectsemploying an optically transmissive material confined between twoopposing broad-area surfaces that extend substantially parallel to eachother. The term “substantially parallel” generally includes cases whenthe opposing surfaces are parallel within a reasonable accuracy. It alsoincludes cases when the body of the material defined by the broad-areasurfaces has a slightly tapered shape or has a slightly varyingthickness across the surface. It yet further includes cases when agenerally planar body of the light guide includes limited areas whereits thickness is different compared to the rest of the light guide.

According to a preferred embodiment of the present invention, the planarlight guide may be exemplified by a transparent plate, sheet, slab,panel, pane, light-transmitting substrate or any suitable sheet-form ofan optically transmissive material, including film thicknesses and rigidand flexible sheet forms. This invention is also applicable to anytwo-dimensional shape variations of the sheet forms, including but notlimited to a square, rectangle, a polygon, a circle, a strip, afreeform, or any combination therein. This invention is furtherapplicable to any three-dimensional shapes that can be obtained bybending the sheet forms accordingly, including but not limited tocylindrical or partial cylindrical shapes, conical shapes, corrugatedshapes, and the like. Opposite ends or edges of such three-dimensionalshapes may be also be connected together to form a continuous surface orsurfaces. For example, a strip of a light guiding strip material can bebent and its edges connected so as to form a loop.

It is also noted that terms such as “top”, “bottom”, “side”, “front” and“back” and similar directional terms are used herein with reference tothe orientation of the Figures being described and should not beregarded as limiting this invention in any way. It should be understoodthat different elements of embodiments of the present invention can bepositioned in a number of different orientations without departing fromthe scope of the present invention. In the context of the description ofa planar light guide and its elements, the term “top” is being generallyused to refer to a primary light emitting side of the light guide andthe term “bottom” is being generally used to refer to the opposite side(which may be emitting or non-emitting) for the sake of convenience ofdescription and not in a limiting sense.

Any numerical range recited herein is intended to include all sub-rangessubsumed therein. For example, a range of 1 to 10 is intended to includeall sub-ranges between and including the recited minimum value of 1 andthe recited maximum value of 10, that is, having a minimum value equalto or greater than 1 and a maximum value of equal to or less than 10,such as, for example, 3 to 6 or 2.5 to 8.5. Any maximum numericallimitation recited herein is intended to include all lower numericallimitations subsumed therein and any minimum numerical limitationrecited in this specification is intended to include all highernumerical limitations subsumed therein. Accordingly, Applicant reservesthe right to amend this specification, including the claims, toexpressly recite any sub-range subsumed within the ranges expresslyrecited herein. All such ranges are intended to be inherently describedin this specification such that amending to expressly recite any suchsubranges would comply with the requirements of 35 U.S.C. § 112, firstparagraph, and 35 U.S.C. § 132(a). Also, unless otherwise indicated, allnumerical parameters are to be understood as being prefaced and modifiedin all instances by the term “about”, in which the numerical parameterspossess the inherent variability characteristic of the underlyingmeasurement techniques used to determine the numerical value of theparameter.

According to some preferred embodiments of the present invention, thereis provided an illumination system employing an optical light guideexemplified by an optically transmissive, broad-area sheet or panel,which may also hereinafter be referred to as a “light guiding sheet”,“light guiding panel” or “LGP”. The LGP is made from a material whichhas a refractive index greater than that of the outside medium and iscapable of guiding light within the panel by means of a Total InternalReflection (TIR) from its opposing broad-area surfaces, provided thatthe internal incidence angles onto either of the surfaces are greaterthan a critical angle of TIR characterizing the broad-area surfaces.

For the purpose of this discussion, the term “incidence angle” of alight ray in relation to a surface generally refers to an angle thatsuch ray makes with respect to a normal to the surface. It will beappreciated by those skilled in the art of optics that, when referringto light or other forms of electromagnetic waves passing through aboundary formed between two different refractive media, such as air andglass, for example, the ratio of the sines of the angles of incidenceand of refraction is a constant that depends on the ratio of refractiveindices of the media (the Snell's law of refraction). The followingrelationship can describe a light bending property of an interfacebetween two refractive media: n_(I) sin ϕ_(I)=n_(R) sin ϕ_(R), wheren_(I) and n_(R) are the respective refractive indices of the materialsforming the optical interface and ϕ_(I) and ϕ_(R) are the angle ofincidence and the angle of refraction, respectively. It will be furtherappreciated that such optical interface can also be characterized by acritical TIR angle which is the value of ϕ_(I) for which ϕ_(R) equals90°. Accordingly, for a surface characterized by a stepped drop inrefractive index along the propagation path of a ray, the incidenceangle may be less than, equal to, or greater than the TIR angle at thegiven surface.

A TIR angle ϕ_(TIR) can be found from the following expression:

ϕ_(TIR)=arcsin(n _(R) /n _(I)·sin 90°)=arcsin(n _(R) /n _(I))  (Equation 1)

In an exemplary case of the interface between glass with the reflectiveindex n_(I) of about 1.51 and air with n_(R) of about 1, ϕ_(TIR) isapproximately equal to 41.5°.

It will be appreciated that, once light is input into the LGP and itspropagation angles permit for TIR to occur at LGP's longitudinal walls,the light becomes trapped in the LGP and can propagate considerabledistances until it is extracted, absorbed or reaches an edge of thepanel, for example.

The present invention will now be described by way of example withreference to the accompanying drawings.

FIG. 1 depicts an embodiment of a wide-area light guide illuminationsystem 900 in accordance with the invention. Light guide illuminationsystem 900 includes a generally planar light guide 800 that is formed bya substantially planar base sheet 10 of an optically transmissivematerial. Light guide 800 may also have a plurality of second sheets ofan optically transmissive material (not shown) attached to the basesheet 10 (such as, for example, sheets 20 described in reference toFIGS. 1-2 of the '865 Application). The orientation of planar lightguide 800 and its components in three-dimensional space may beconveniently described using orthogonal reference axes X, Y, and Z (seeFIG. 1) which also define orthogonal reference planes XY, XZ, and YZ.

Sheet 10 has a rectangular configuration and is defined by opposingbroad-area surfaces 11 and 12 and four edge surfaces 13, 14, 15 and 16.Surfaces 11 and 12 extend parallel to each other and extend broadly bothlongitudinally and laterally along the X and Y axes so as to form aplanar sheet form that is parallel to the XY plane. Opposing edgesurfaces 13, 14 are parallel to each other, extending parallel to the XZplane, and opposing edge surfaces 15, 16 are likewise parallel to eachother, extending parallel to the YZ plane. Sheet 10 has a non-zerothickness which may be conventionally measured along the Z axis orcoordinate.

Sheet 10 is preferably formed from a highly transmissive, soliddielectric material and is configured to guide light both longitudinallyand laterally using optical transmission through the material and TIRfrom opposing surfaces 11 and 12. Surfaces 11 and 12 are preferablyoptically smooth and polished to high gloss. Edge surfaces 13, 14, 15and 16 may also be polished and configured for reflecting light by meansof TIR with high efficiency. One or more edge surfaces 13, 14, 15 and 16may also be covered with a specularly reflective mirror or a diffusereflector. For example, any of the edge surfaces may be coated with ametallic layer (e.g., aluminized or silvered). In another example,strips of highly reflective material, such as a metallized film or foilmay be applied to any of the edge surfaces 13, 14, 15 and 16. Anothersuitable exemplary type of the reflective materials may also be a white,light diffusing tape.

Suitable materials for making sheet 10 may include various dielectricmaterials in the form of a wide-area, highly transparent sheet or film.Materials that may be particularly suited for making sheet 10 includebut are not limited to water-clear (low-iron) glass, Poly(methylmethacrylate) (PMMA or acrylic), polycarbonate, styrene, cured urethane,polyester, silicone, and the like.

Sheet 10 has a length L₁₀ and a width W₁₀ that can be considerably lessthan length L₁₀. According to different embodiments, width W₁₀ is lessthan length L₁₀ by at least 1.5 times, at least 2 times, at least 3times, at least 5 times, and at least 10 times. The thickness of sheet10 can be make sufficiently low to make it flexible. According to oneembodiment, the thickness of flexible sheet 10 can be in the range of0.3 mm to 2.5 mm and more preferably from 0.5 mm to 1.5 mm so that thesheet could be flexed and handled with relative ease without breaking oraffecting its structural integrity.

Light guide illumination system 900 further includes a plurality ofcompact solid-state light sources exemplified by LEDs 2. LEDs 2 areprovided in four linear arrays or strips where each array or strip isconfigured to illuminate the respective edge of sheet 10 (edge surfaces13, 14, 15 and 16). In other words, all of the edges of sheet 10 areconfigured as light input edges. Within each array or strip, LEDs 2 arepositioned adjacent and optically coupled to the respective edge surfacesuch that the amount of light that is not coupled to light guide 800(light spillage) is minimized.

Surface 11 of sheet 10 includes a plurality of discrete light extractionfeatures 8 forming a two-dimensional light extraction pattern 101 andconfigured to extract light from light guide 800. According to oneembodiment, light extraction pattern 101 may occupy substantially theentire exposed area of light guide 800. It may also extend all the waylongitudinally between opposing edge surfaces 13 and 14 and laterallybetween opposing edge surfaces 15 and 16.

According to one embodiments, light extraction features 8 are formedonly in surface 11 while opposite surface 12 can be substantially freefrom light extraction features 8. According to one embodiments, lightextraction features 8 are formed only in surface 12 while surface 11 canbe substantially free from light extraction features 8. According to oneembodiments, light extraction features 8 are formed in both opposingsurfaces 11 and 12.

While some of the paragraphs below may describe embodiments ofillumination system 900 primarily referring to light extraction features8 being formed in surface 11, it should be understood that thisinvention is not limited to this and that the same description can beapplied to the cases when light extraction features 8 are formed insurface 12 or in both surfaces 11 and 12. Furthermore, it should beunderstood that these embodiments are amenable to various modificationsand alternative forms, for example, in which light extraction features 8are formed in other surfaces that are parallel or near-parallel tosurfaces 11 and 12, especially when light guide 800 includes two or morelayer of optical transparent materials. According to some embodiments,light guide 800 may be formed by two sheets of optically transmissivematerials that are attached (and optionally bonded) to each other. Atleast some of light extraction features 8 may be formed in or on theinside broad-area surface of one or both of the sheets, so that lightextraction features 8 become embedded into the material of light guide800 when the sheets are bonded together.

In operation, an exemplary light ray 133 emitted by one of LEDs 2optically coupled to light input edge 13 is propagated within sheet 10in a waveguide mode until it is extracted by one of light extractionfeatures 8 and is directed out and away from light guide 800. Dependingon the location, configuration and optical properties of extractionfeatures 8, as well as probability, ray 133 may exit from either surface11 or 12. Depending on the same factors, ray 133 may exit from surface11 or 12 at a right angle or at an oblique angle with respect to thesurface plane.

According to one embodiment, light extraction pattern 101 may have auniform average areal density or coverage with a randomized spacingbetween individual light extraction features 8, for example, asillustrated in FIG. 1. The spacing may be randomized, for example, suchthat adjacent individual light extraction features 8 are spaced fromeach other by spacing distances that deviate from an average spacingwithin a sampling area by no less than a minimum spacing distance and nomore than a maximum spacing distance.

A spacing distance (SPD) may be ordinarily defined as a distance betweengeometrical centers of individual light extraction features 8, e.g., asschematically illustrated in FIG. 2. According to different embodiments,a minimum spacing distance SPD_(MIN) characterizing a particularsampling area of light extraction pattern 101 may be 0.1, 0.25, 0.5,0.75, or 0.9 times an average spacing distance (SPD_(AVG))characterizing the same sampling area. According to differentembodiments, a maximum spacing distance SPD_(MAX) characterizing aparticular sampling area of light extraction pattern 101 may be 1.2,1.5, 2, 2.5, 3, 5 or 10 times the average spacing distance SPD_(AVG).

A separation distance (SED) in relation to a pair of adjacent individuallight extraction features 8 may be defined as the shortest distanceconnecting the respective outlines or boundaries of such adjacent lightextraction features (FIG. 2). In an exemplary case of two adjacent lightextraction features 8 each having a round outline or aperture with adiameter d, separation distance SED may be defined as the spacingdistance SPD minus diameter d. Accordingly, depending on the values ofthe spacing distance and diameter d, the separation distance may havenegative and positive values, and may also be zero when SPD=d.

The definition of separation distance SED may also be generalized to thecases where light extraction features 8 have shapes other than round.For example, an average size or an average diameter of light extractionfeatures 8 may be used in place of diameter d to define separationdistance SED. The average diameter or size of individual lightextraction feature 8 may be defined as an average length of diametersmeasured at predefined angular intervals around a centroid of the shaperepresenting such light extraction feature 8. For example, the angularintervals can be 1°, 2° 5°, 10°, 20°, 30° or 45°.

According to one embodiment, separation distance SED characterizing apair of adjacent light extraction features 8 having round or non-roundshapes may be considered negative when these adjacent light extractionfeatures 8 (or their apertures or outlines) substantially overlap (FIG.4), zero when they (or their apertures or outlines) are disposed incontact or extremely close to each other (FIG. 3), and positive whentheir outlines or apertures neither overlap nor contact each other (FIG.2).

According to one embodiment, separation distance SED between at leastsome adjacent light extraction features 8 is negative, e.g., lightextraction features 8 are substantially overlapping. According todifferent embodiments, the overlapping light extraction features 8 mayoverlap by 10% or more, 20% or more, 30% or more, 50% or more, 75% ormore, or 80% or more. According to an aspect of the embodiments in whichtwo or more light extraction features 8 overlap, such overlapping lightextraction features 8 may be cumulatively considered a larger singlelight extraction feature 8. In an exemplary case where each of theoverlapping light extraction features 8 is formed by an individual dropof light scattering ink, the resulting, larger light extraction featuremay ordinarily have a total volume that is a whole multiple of thevolume of the individual drops forming individual light extractionfeatures 8. For example, individual microdrops having a volume of 4picoliters may form larger drops having volumes of 8 picoliters, 12picoliters, 20 picoliters, 40 picoliters and so on.

According to one embodiment, individual light extraction features 8 maybe formed by depositing two or more microdrops of a light scattering inkto the same location of surface 11. According to one embodiment,individual light extraction features 8 may be formed by depositing twoor more microdrops with slight offset with respect to each other. It ispreferred that each microdrop deposited to surface 11 is at leastpartially or completely cured to a high-viscosity or substantially solidstate before depositing a next microdrop on top of it. The dropletdeposition process can be repeated to gradually build up a prescribedthickness and/or size of the respective microdot in stepped incrementsbased on the volume of individual droplets.

According to one embodiment, separation distance SED between at leastsome adjacent light extraction features 8 is zero or near zero. In otherwords, the light extraction features 8 are contacting each other ortheir apertures are very close to each other or have a very smalloverlap (e.g., within less than 10% of the average diameter of each ofthe adjacent light extraction features 8).

According to different embodiments, separation distance SED between atleast some adjacent light extraction features 8 is greater than 25%,greater than 50%, greater than 100%, greater than 150%, or equal to orgreater than 200% of diameter d or the average diameter characterizingthe light extraction features 8. According to different embodiments,separation distance SED between at least some adjacent light extractionfeatures 8 is less than 0.9, less than 0.75, less than 0.5, or less than30% of diameter d (in case of round-apertures) or the average diameter(in case of non-round-apertures) characterizing light extractionfeatures 8.

For the purpose of measuring average values of the spacing distances SPDand/or separation distances SED, a suitable sampling area may be definedas a relatively small-size area of surface 11, at a particular locationof the surface, which includes at least 100 individual light extractionfeatures 8. According to different embodiments, an average separationdistance SED_(AVG) between adjacent light extraction features 8 withinthe sampling area is greater than 25%, greater than 50%, greater than100%, greater than 150%, or equal to or greater than 200% of diameter dor the average diameter characterizing the light extraction features 8.According to different embodiments, an average separation distancebetween adjacent light extraction features 8 within the sampling area isless than 0.9, less than 0.75, less than 0.5, or less than 30% ofdiameter d (in case of round-apertures) or the average diameter (in caseof non-round-apertures) characterizing light extraction features 8.

According to one embodiment, light extraction features 8 may be formedby repeatedly depositing a number of individual droplets with a slightoffset from each other. The offset can be selected to be less than theprevalent diameter of the individual microdots formed by each droplet,creating at least partial overlap for the resulting micro- ormacro-dots. According to one embodiment, the direction of the offset canbe maintained for depositing a series of individual droplets such that asingle elongated light extraction feature 8 in the form of a straightline can be formed. According to one embodiment, the direction of theoffset can be varied so as to produce continuous curved lines or acombination of straight and curved lines or line segments. The width ofthe lines or line segments produced by this method may be controlled,for example, by the volume and viscosity of each droplet, wettability ofsurface 11, temperature of the ink or the substrate (sheet 10), numberof drops, amount of the offset, as well as depositing some of themicrodots with a perpendicular offset from the intended center of theline or line segment. Multiple straight and/or curved lines may bebranched at one or multiple locations, e.g., to produce a tree-likestructure.

Light extracting features 8 may include any suitable two- orthree-dimensional optical elements or surface features configured forintercepting and extracting light from sheet 10. Light extractingfeatures 8 may be configured to extract light by means of scattering,reflection, refraction, deflection, diffraction, absorption with thesubsequent re-emission, or any combination thereof.

Light extracting features 8 may be further configured to extract lightwhile changing one or more properties of light. Exemplary properties oflight that may be changed by light extracting features 8 include but arenot limited to a wavelength, polarization, spectral distribution,angular and/or spatial distribution, and dispersion. For example, eachlight extracting features 8 may include a color pigment that receiveswhite color and either filters out certain wavelengths or converts thereceived light to a different color. For example, each light extractingfeatures 8 may include a color pigment that is configured for convertinga white color to a different color, e.g. red, green or blue. In afurther example, each light extracting features 8 may include afluorescent material (e.g., a phosphor) that that is configured toreceive a blue color light and convert it to a white light. In a furtherexample, light extracting features 8 may include a light-scatteringmaterial that disperses the incident light over a wide angular range. Afluorescent material can be combined or mixed with a light scatteringmaterial. For example, particles of a phosphor material may be mixedtogether with light-scattering particles into the ink used to producelight extraction features 8.

According to one embodiment, light extraction pattern 101 may have anon-uniform average areal density (or coverage) of individual lightextraction features 8 at different locations of surface 11. The spacingbetween individual light extraction features 8 may be varied accordingto a regular or irregular pattern. Furthermore, light extraction pattern101 may be segmented into multiple smaller-area light extractionpatterns that are either separated from each other or have differentoptical properties or distribution densities of light extractionfeatures. These smaller-area light extraction patterns may have distinctboundaries. They can also be separated from each other by spacing orseparation areas that are generally free from light extraction features8. According to one embodiment, the spacing or separation areas may bedistributed over the area of surface 11 according to an orderedgeometrical pattern, such as for example, an array of parallel bands.

FIG. 5 illustrates an embodiment of system 900 in which light extractionpattern 101 is segmented into multiple sub-patterns, as indicated byareas 55. Those sub-patterns (areas 55) are separated from each other byseparation areas 54 that are generally free from light extractionfeatures 8. Separation areas 54 are configured in the form of aperpendicular grid of narrow bands that extend all the way betweenopposing edges 13 and 14 and 15 and 16. More specifically, the bandsrepresenting separation areas 54 are arranged into two parallel arraysthat intersect with each other at a right angle. The parallel bands ofthe first array extend perpendicular to light input edges 13 and 14 andthe parallel bands of the second array extend parallel to light inputedges 13 and 14. Each band or strip representing an individualseparation area 54 may have a width W₅₄ that is less than the length orwidth of patterned areas 55.

According to some embodiments, it may be preferred that width W₅₄ ofeach separation area 54 is at least several times greater than prevalentspacing distances SPD between light extraction features 8 in areasimmediately adjacent to the respective separation area 54. According tosome exemplary embodiments, width W₅₄ can be at least 3 times, 5 times,10 times, 20 times, 50 times, and 100 times greater than an average SPDcharacterizing the distances between light extraction features 8 in anadjacent patterned area 55. According to some embodiments, it may bepreferred that the size of each area 54 is at least several timesgreater than a prevalent or average spacing distances SPD between lightextraction features 8 within such area, e.g., at least 3 times, 5 times,10 times, 20 times, 50 times, 100 times, or 1000 times.

The operation of light guide illumination system 900 of FIG. 2 isillustrated by the example of an exemplary light ray 134. Ray 134 isemitted by one of the LEDs 2, propagated through light guide 800 (sheet10) and extracted using individual light extraction feature 8 of lightextraction pattern 101 such that ray 134 further propagates towards aprescribed direction (e.g., towards a viewer or an object to beilluminated).

According to one embodiment, separation areas 54 may be configured tosuppress or otherwise significantly limit the rate of light extractionin those areas such that the guided light can only be extracted from oneof areas 55. This may be useful, for example, in order to create avisually distinct appearance of light guide 800 when illuminated by LEDs2. In a further non-limiting example, separation areas 54 may beconfigured to provide some visual transparency of light guide 800 evenwhen it is illuminated, regardless of the density and/or light-blockingoperation of patterned areas 55.

In a yet further non-limiting example, separation areas 54 may bepositioned in areas of sheet 10 which are covered by an opaque material.For instance, light guide 800 may be associated with a grid orreflective or light absolving members. Such a grid may be exemplified byan egg-crate lighting diffuser or a grid of parabolic louvers and theopaque members may be represented by the louvers or the walls of theegg-crate diffuser structure. Since extracting and emitting light in theareas where the opaque grid members are located could result in a lossof efficiency (e.g., due to light absorption or reflection by the gridmembers) suppressing the light emission in those areas by providingseparation areas 54 may help enhance the overall efficiency of thelighting device (system 900).

According to some embodiments, spacing areas 54 may also be used forattaching other optical elements to light guide 800, such as, forexample, light coupling elements disclosed in the '361 and '666Publications or light guiding elements disclosed in the '865Application.

According to one embodiment, the arrangements and optical properties ofindividual light extraction features 8 may generally be the same orsimilar for each area 55. According to one embodiment, the arrangementsand/or optical properties of individual light extraction features 8 maybe the different for different areas 55. For example, the areal density,spatial distribution, color properties, sizes and/or shapes of lightextraction features 8 may be made variable from one area 55 to another.

According to an aspect of the embodiment of FIG. 5, surface 11 has lightextracting areas (areas 55) that are alternating with spacing orseparation areas (areas 54) in a repeating pattern. According to oneembodiment, width W₅₄ is constant along the entire length of eachindividual separation area 54. According to one embodiment, width W₅₄ isvariable along the length of the individual separation area 54.According to one embodiment, width W₅₄ may also differ from oneseparation area 54 to another.

According to one embodiment, each light extracting feature 8 is formedby a relatively small dot (a microdot) of a highly diffusely reflective,light scattering material deposited to surface 11. The microdots may bedistributed over surface 11 according to an ordered or randomtwo-dimensional pattern. Suitable materials for light extractingfeatures 8 may include white inks or paints having a reflectance of atleast 80% in the visual spectrum, preferably having at least 85%reflectance, even more preferably at least 90% reflectance, and stilleven more preferably at least 95% reflectance. Light scattering dots maybe formed by white inks that are radiation-curable (in particular,UV-curable), aqueous (water-based) or solvent-based. When LEDs 2 areconfigured to emit light in a particular wavelength range, the inkmaterial should preferably have a reflectance greater than 85%, 90% or95% in that wavelength range.

According to one embodiment, individual light extraction features 8 mayhave a volume between 1 picoliter and 10 picoliters. According to oneembodiment, each or at least some of individual light extractionfeatures 8 may have a volume of about 1 picoliter. According to oneembodiment, individual light extraction features 8 have a volume ofabout 3 picoliters. According to one embodiment, individual lightextraction features 8 have a volume of about 4 picoliters. According toone embodiment, individual light extraction features 8 have a volume ofabout 5 picoliters. According to one embodiment, individual lightextraction features 8 have a volume of about 10 picoliters.

The microdots can be printed on surface 11 using a flatbed orroll-to-roll material deposition printer, a UV printer, an ink-jetprinter, a sublimation printer, or a screen printer, for example.According to one embodiment, the white ink may include nanoparticles oftitanium dioxide, strontium sulfide, zinc sulphide, zink oxide, or othertype of white, high-reflectance powder suspended in a liquid resin orsuspension which viscosity is suitable for the selected type of surfacedeposition technique (e.g., UV printing). The nanoparticles may beformed by any type of a high-refractive-index material (which betransparent) and may be configured to scatter light primarily usingdiffraction, at least in one preselected wavelength range. According toone embodiment, light extraction features 8 may include materials withspecific color-filtering properties (e.g., pigmented inks or fluorescentinks) and can change the color of light.

According to one embodiment, the light-scattering microdots are formedby a UV-curable ink that includes nanoparticles of opticallytransmissive, high-refractive-index material suspended in a translucentor, more preferably, highly transparent polymerizable binder materialthat has a significantly lower refractive index than that of thenanoparticles. Examples of the transparent polymerizable binder materialbinder materials include various acrylates and their derivatives (e.g.,epoxy acrylates, polyurethane acrylates and polyester acrylates)obtained by reacting an acrylate with a suitable epoxide, urethane orpolyester resins. In one embodiment, the binder material may alsoinclude a polyester resin or polyurethane resin mixed with aUV-polymerizable reagent. According to one embodiment, the bindermaterial may include acryl acid ester (e.g., 40-60% by weight) and1,6-Hexanediol diacrylate (e.g., 20-30% by weight).

According to one embodiment, at the time of printing (or otherwisedeposition to a surface of light guide 800), the uncured ink shouldpreferably have a viscosity in the range of 10 to 150 centipoise (cP).If the ink has a higher viscosity at room temperature (25° C.), it maybe heated before surface deposition to bring the viscosity down to theprescribed range. According to one embodiment, the viscosity of theuncured ink at room temperature is between 10 cP and 30 cP. According toone embodiment, the viscosity of the uncured ink at room temperature isbetween 5 cP and 15 cP. According to one embodiment, the viscosity ofthe uncured ink at room temperature is from 5 cP to 25 cP.

According to one embodiment, light extracting features 8 may havephosphorescent or fluorescent properties. For example, the resin orsuspension used to print light extraction features 8 on surface 11 mayinclude a fluorescent material or phosphor that converts a shorterwavelength of light in the ultraviolet (UV) or visible spectrum into oneor more longer wavelengths in the visible range. Such phosphorescentmaterial can be configured to absorb light in a first wavelength andre-emit light in a second wavelength which is different than the firstwavelength. According to one embodiment, it is preferred that the secondwavelength is greater than the first wavelength. By way of example, thematerial may be configured to absorb at least a portion of blue lightemitted by some types of LEDs and re-emit the energy of such blue lightin the form of perceptibly white light. By way of example and notlimitation, the fluorescent material may be configured to convert 350nm-400 nm UV light from a “black light” into visible wavelengths (e.g.,500 nm-600 nm).

Light extracting features 8 may be distributed over the designatedarea(s), e.g., areas 55, according to an ordered or random pattern.According to one embodiment, such pattern may be formed by a twodimensional array of rows and columns. In one implementation, everyother row or every other column may be shifted relatively to theadjacent rows or columns so as to form a staggered array or rows orcolumns. According to one embodiment, the positions of individual lightextracting features 8 may be randomized within an otherwise orderedpattern. According to one embodiment, light extraction features 8 aredistributed according to a high-density pattern and have a cumulativearea that approximates the exposed area of surface 11. According to oneembodiment, substantially the entire exposed area of surface 11 may becoated by a continuous layer of a semi-opaque light diffusing material,such as non-absorbing white ink or bulk scattering particles suspendedin a polymeric material, for example. According to one embodiment, oneor more areas 55 may be substantially covered with a continuous layer ofa semi-opaque light diffusing material.

According to one embodiment, light extraction features 8 are formed by alight scattering or light diffusing film that is attached to surface 11in the respective areas (e.g., sections 55). Such film should preferablyhave a hemispherical reflectance of at least 85%, more preferably atleast 90%, and still more preferably at least 95%.

LED chips employed in LEDs 2 may be configured to emit a blue light.Light extracting features 8 may be configured to change the lightemission spectrum upon interaction with blue light propagating in lightguide 800. For example, a YAG phosphor may be employed in lightextracting features 8 to convert such blue light to a white light. Thephosphor material may be mixed with silicone or other encapsulationmaterial. Light extraction features 8 may be deposited directly tosurface 11 in a liquid form, for example, by printing, spraying,dispensing, coating or other suitable liquid material depositionmethods.

According to one embodiment, light extraction features 8 are formed bylight-deflecting or light-diffusing surface microstructures formed in oron surface 11. The microstructures may include ordered or random surfacerelief features formed, for example, by means of etching, embossing,laser ablation, sanding, micromachining, micro-replication and any othermethod suitable for producing the desired surface texture or relief.

According to some embodiments, light extraction features 8 may be formedby micro-cavities formed in surface 11. Each micro-cavity may have theshape of a lens, a prism, a blind hole, or can be simply a microscopicdiscontinuity in surface 11 allowing some light to escape from lightguide 800 in the respective location. The size of individual lightextraction features may range from submicron sizes up to severalmillimeters. According to one embodiment, the size of individual lightextraction features 8 is between 1 micrometer and 25 micrometers.According to one embodiment, the size of individual light extractionfeatures 8 is between 20 micrometers and 100 micrometers. According toone embodiment, the size of individual light extraction features 8 isbetween 40 micrometers and 100 micrometers. According to one embodiment,the size of individual light extraction features 8 is between 40micrometers and 200 micrometers. According to one embodiment, the sizeof individual light extraction features 8 is sufficiently small and thespacing between individual features is sufficiently large so that lightguide 800 has a substantially transparent appearance at least whenviewed from a normal viewing direction.

According to one embodiment, light extraction features 8 are formed in aseparate film or thin-sheet material which is then applied to surface 11with a good optical contact and preferably with refractive indexmatching. According to one embodiment, light extraction features 8 maybe formed in surface 12. According to one embodiment, light extractionfeatures 8 may be formed in both surfaces 11 and 12, for example, toenhance the light extraction rate without allowing individual lightextraction features 8 to be too close to each other.

According to some embodiments, referring to FIG. 5, certaincharacteristics of light extraction features 8 or their two-dimensionalpattern in one area 55 may be different from those of another (e.g.,adjacent) area 55. For example, the geometric patterns, relative areasoccupied by light extraction features 8, the spacing between adjacentlight extraction features 8, the size, thickness, reflectance,absorption, color or fluorescent properties of various light extractionfeatures 8 can be different in different parts of surface 11.

The properties of light extraction features 8 may also vary graduallyacross surfaces 11 and/or 12. According to one embodiment, lightextraction features 8 of surface 11 may be formed by one type of lightdeflecting elements (e.g., inkjet-printed microdots) while lightextraction features 8 of surface 12 may be formed by a different type oflight deflecting elements (e.g., surface microstructures formed bymicroimprinting or hot embossing).

Light extraction features 8 are configured to progressively extractlight propagating in sheet 10 and result in a substantially uniformlight emission from either one or both surfaces 11 and 12. In order toachieve a uniform emission, the two-dimensional pattern of lightextraction features 8 can have a variable spatial density in differentareas. According to one embodiment, the density should increase with thedistance from LEDs 2. Such a variable spatial density can be a functionof the size and thickness of sheet 10 and can be determined from opticalraytracing or actual experiments with different-density patterns.

According to one embodiment, areas 54 may include light extractionfeatures 8 having a much lower areal density compared to adjacent areas55. The areal density of light extraction features may also vary withineach of the areas 54 and 55.

A uniformity U of luminance of a broad-area surface of sheet 10 (e.g.,surface 11 or surface 12) may be defined by the following relationship:U=1−(L_(PEAK)−L_(AVG))/L_(AVG), where L_(PEAK) is a peak luminance andL_(AVG) is an average luminance characterizing the surface. The peakluminance may be measured using spot measurements at different locationsof the broad-area surface using a spot luminance meter. The samplingarea for spot measurements may be defined by a circular areacharacterized by a radius that is much smaller than the X and Ydimensions of light guide 800. A preferred size of the sampling area mayalso be defined by the characteristics of the measurement tool, theoverall size of the panel or the intended application (for example, theanticipated viewing distance). In other words, the spot measurementsshould preferably have sufficient granularity to measure surfaceluminance variations across the light-emitting surface of light guide800.

According to one embodiment, luminance uniformity U of light guide 800(as measured at either one of surfaces 11 and 12) is at least 70%, morepreferably at least 80%, even more preferably at least 85%, and yet evenmore preferably at least 90%. According to one embodiment, a differencebetween an average luminance of different areas 55 is less than 30%,more preferably is less than 25%, even more preferably is less than 20%,even more preferably is less than 15%, and still even more preferably isless than 10%.

According to one embodiment, light guide 800 may be formed by twodistinct light guiding layers which represent different opticallytransmissive sheets. According to different implementations, lightextraction features 8 may be formed in one or both of the sheets (lightguiding layers).

It should be understood that light sources illuminating the light inputedges (such as opposing edge surfaces 13 and 14) are not limited tolight emitting diodes (LEDs) and may include any suitable individual ormultiple light sources of any known type, including but not limited to:fluorescent lamps, incandescent lamps, cold-cathode or compactfluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide,electroluminescent lamps or sources, field emission devices, lasers,etc. Each individual light source may have a linear configuration andinclude a single linear light-emitting element or a relatively smallnumber of linear light-emitting elements. Each light source may alsohave two or more compact light emitting elements incorporated into alinear array. When the light source includes multiple light emittingelements, each of the light emitting elements may have a compact shapeor an extended two-dimensional or one-dimensional (elongated) shape.

According to one embodiment, at least one of the light sources opticallycoupled to sheet 10 includes a laser source emitting a highly collimatedbeam of light. Suitable examples of a highly collimated beam includelight beams having a full width at half-maximum (FWHM) divergence angleof less than 30°, less than 25° degrees, less than 20° degrees, lessthan 15° degrees, less than 10° degrees, and less than 5° degrees.According to one embodiment, at least one of the light sources opticallycoupled to sheet 10 includes an LED or laser source emitting amoderately collimated beam of light having a FWHM divergence angle ofless than 60°, less than 50° or less than 45°. According to someembodiments, the above-referenced FWHM divergence angles may be definedand measured in a plane that is perpendicular to a prevalent plane ofsheet 10 (e.g., in the YZ plane when light is input through edge surface13).

LEDs 2 may be further associated with integrated or external optics suchas collimating or light-redistributing lenses, mirrors, lens arrays,mirror arrays, light diffusers, waveguides, or optical fibers. Whenmultiple light emitting elements are employed, each of the lightemitting elements may be provided with individual optics. Alternatively,a single linear optic may be provided for the entire array to collimatelight or otherwise shape the emitted beam in a plane which isperpendicular to the longitudinal axis of the array.

Light guide illumination system 900 may include a cover of housing 64configured to encase LEDs 2 and optionally portions of sheet 10 adjacentto the light input edge(s). Such housing 64 may have different functionsincluding but not limited to structural, protective (from dust,moisture, elements, impact, etc.) and/or aesthetic.

The overall dimensions of wide-area light guide illumination system 900,the size and shape of sheet 10, the types of LEDs 2 as well as theirnumber, spacing and nominal power may be selected based on the targetapplication. According to one embodiment, light guide illuminationsystem 900 may be configured as a flat-panel lighting luminaire.According to another embodiment, it may be configured as a backlight ofan LCD display. According to a yet another embodiment, it may beconfigured as a backlight for an illuminated sign, artwork or imageprint. According to a yet another embodiment, it may be configured as anillumination system (e.g., backlight or planar-panel light) for a planarpanel photobioreactor.

Areas 55 may have different rectangular shapes and arranged on surface11 according to a different pattern compared to those of FIG. 5.Furthermore, light extraction pattern 101 may include a mix of differentshapes and sizes of areas 54 and 55. This is further illustrated in FIG.6, which schematically depicts other geometrical configurations of areas55 within light extraction pattern 101 and also schematically depictsother dimensions and arrangements of LEDs 2.

According to one embodiment, each of area 55 may have a hexagonal shape.An exemplary arrangement of such hexagonal areas 55 within lightextraction pattern is schematically shown in FIG. 7. It should beunderstood that such pattern of hexagonal light extraction areas 55 mayextend continuously over broad-area surface 11 both longitudinally andlaterally (along the X and Y directions) to cover sufficiently largeareas. The sizes and density of light extraction features 8 within eachhexagonal area 55 may be selected such that an average luminance ofdifferent parts of pattern 101 is about the same or similar, e.g.,within 20% of the average luminance produced by the entire patternedsurface of sheet 10.

According to one embodiment, LEDs 2 may include individually digitallyaddressable RGB LEDs. Such individually digitally addressable RGB LEDs 2may be selectively turned on and off or dimmed to illuminate selectareas of stepped light guide illumination system 900 in differentbrightness and/or color.

According to one embodiment, individual light extraction features 8 orgroups of light extraction features 8 are individually controllable andcan change their color or light extraction properties in response to anexternal factor (e.g., supplied voltage, magnetic field, electric field,static electricity, illumination by an external source of light,mechanical or optical contact with an external object, etc.). Suchindividually controllable light extraction features 8 or groups may beselectively turned on and off or dimmed to illuminate select areas oflight guide illumination system 900 in different brightness and/orcolor.

According to one embodiment, individual areas 55 may be configured andindividually controlled as individual “pixels” within a largeilluminated LED display. Such LED display may incorporate hundreds andthousands of areas 55. For example, each pixel including a singleindividually controllable area 55 may have a size from 0.5 mm to 5 mm orfrom 1 mm to 10 mm.

FIG. 8 schematically illustrates an exemplary light extraction pattern102 including light extraction features 8 formed in areas 55 and lightextraction features 9 formed in areas 54 that are alternating with areas55 along the Y axis. The areal density of light extraction features 9within areas 54 of pattern 102 is much lower than the areal density oflight extraction features 8 within areas 55 of the pattern. The spatialdistributions of light extraction features 8 within each area 55 has avariable density. At least some areas 55 have a positive gradient of theareal density towards a central axis. Similarly, the spatialdistributions of light extraction features 9 within each area 54 has avariable density with a positive gradient of the areal density towards acentral axis. The areal density of light extraction features 8 and/or 9may be expressed, for example, in terms of the number of respectivelight extraction features per unit area. According to an aspect, lightextraction pattern 102 includes alternating bands having different arealdensities of light extraction features 8 and 9. Furthermore, the arealdensity is variable within each band.

A relative surface area of light extraction features 8 and/or 9 at anyparticular location of surface 11 and/or surface 12 may be defined as asum of the individual areas of light extraction features 8 and/or 9within a selected sampling area divided by the total area of thesampling area. For example, a relative surface area of 0.5 correspond toone-half of the sampling area being occupied by the light extractionfeatures (50% areal coverage). A relative surface area equal to onemeans that the light extraction features occupy 100% of the samplingarea, with no spaces. Depending on the size and shape of individuallight extraction features, the spatial density and relative surface areamay be bound by various predefined relationships.

According to one embodiment, light extraction patterns of features 8and/or 9 are characterized by spacing distances SPD which progressivelydecrease with a distance from LEDs 2 at least within some samplingareas. According to one embodiment, spacing distances SPD progressivelyincrease with a distance from LEDs 2 within at least some samplingareas. According to one embodiment, the light extraction patterns oflight extraction features 8 and/or 9 are characterized by spacingdistances SPD which progressively increase with a distance from LEDs 2within at least a first sampling area and progressively increase with adistance from LEDs 2 within at least a second sampling area that isdifferent from the first sampling area.

FIG. 8 illustrates varying the spatial density of light extractionfeatures 8 and 9 (and, hence, varying the relative area occupied bylight extraction features 8 and 9) by varying the spacing betweenindividual light extraction features. However, it should be understoodthan the relative area may also be varied by varying the size ofindividual features 8 and/or 9, even at a constant spacing. For example,increasing the area of each light extraction feature 8 or 9 by two timeswithin a particular sampling area will increase the relative area ofsuch features within the sampling area by two times.

According to one embodiment, LEDs 2 may be side-emitting LEDs. In someimplementations, the side-emitting LEDs may be attached directly tosurface 11 or 12 of sheet 10 (e.g. glued using a two-sided adhesivetransfer tape). Examples of side emitting LEDs that may be suitable forLEDs 2 include but are not limited to Micro SIDELED product seriescommercially available from OSRAM (e.g., LW Y87C, CUW Y3SH.B1 and LWY1SG models of white LEDs or LB Y8SG model of blue LEDs) or modelsNS2W364G and NS2W266G of white side-emitting LEDs manufactured byNichia.

It is noted, however, that the embodiments of wide-area light guideillumination system 900 described herein may also be adapted to usingother types and form factors of side-emitting LEDs and may further beadapted to many different types and configurations of LEDs, includingsquare, round or rectangular top-emitting LEDs of various architectures.Furthermore, non-LED light sources can be used in place of LEDs 2, suchas, for example, lasers, fluorescent lamps, incandescent lamps,gas-discharge lamps, and OLEDs. LEDs 2 may incorporate LED arrays orarrays of LED die assembled within a single package. Suitable examplesof such LEDs as well as related methods of LED coupling to light guides(waveguides) are disclosed, for example, in the '666 Publication.Additional exemplary embodiments of LEDs and light coupling structuresthat can be used to input light into light guide 800 are disclosed inthe '865 Application. “Example 1” in the '865 Application furtherdiscloses an exemplary configuration of light extraction patterns forobtaining a substantially uniform light emission from the entire lightemitting area of a planar-type light guide, which can be applied forpatterning light guide 800 of this invention, according to at least someembodiments.

FIG. 9 schematically depicts a portion of light guiding sheet 10 andindividual light extraction feature 8 exemplified by a fully-cured,solidified drop (microdot) of a UV-curable ink deposited to surface 11.It is noted that the illustrative example of FIG. 9 may also be appliedto configuring light extraction features 9 above and may further beapplied to embodiments in which light extraction features 8 and/or 9 areformed in surface 12.

Referring FIG. 9, the UV-curable ink forming light extraction feature 8includes a highly transparent, UV-reactive binder 37 and a suspension ofhigh-refractive-index, light-scattering particles 33. A suitable exampleof light-scattering particles 33 having a high refractive index issubmicron-sized particles of titanium dioxide.

Particles 33 are about evenly distributed throughout the volume ofUV-reactive binder 37 and provide volumetric bulk scattering propertiesfor light extraction feature 8. Some particles 33 may form agglomerates35 in which a number of particles 33 may be disposed in contact witheach other or at a very close distance to one another. Such distance canbe much smaller than the average distance between individual particles33 in binder 37. Agglomerates 35 may form two- or three-dimensionalstructures that have sizes from a fraction of the micrometer to severalmicrometers.

According to one embodiment, light extraction feature 8 of FIG. 9 mayhave an irregular elongated shape having a length L₈ and a maximumheight H₈. Maximum height H₈ may also be referred to as a maximumthickness of light extraction feature 8. A transverse width of theelongated light extraction feature 8 may be 1.2 times, 1.5 times, 2times, 2.5 times, 3 times, 5 times or 10 times less than length L₈. Anaspect ratio (length to width ratio) may vary randomly from one lightextraction feature 8 to another. In one embodiment, elongated lightextraction features 8 may be arranged in groups having generally thesame orientation of a longitudinal axis. In one embodiment, theorientations of elongated light extraction features 8 may be randomwithin a predefined angular range. The angular range may be 10 degrees,30 degrees, 45 degrees, 60 degrees, 90 degrees, 180 degrees, and 360degrees (e.g., with a completely random orientation).

A surface 36 of light extraction feature 8 that is exposed to air has amicrostructured surface including random microstructures 34.Microstructures 34 produce a non-negligible surface roughness that ismuch greater than the roughness of surface 11 and that contributes torefractive or diffractive light scattering or dispersion produced bylight extraction feature 8. The surface roughness may be characterizedaccording to an American National Standard ASME B46.1-2009.

According to different embodiments, an RMS surface roughness parameterR_(q) of surface 36 is greater than 30 nanometers, greater than 40nanometers, greater than 60 nanometers, approximately equal to orgreater than 100 nanometers, and approximately equal to or greater than200 nanometers. At the same time, it may be preferred that the RMSsurface roughness parameter R_(q) of surface 36 measured along the samesampling length is one of the following: less than 1 micrometer, lessthan 0.5 micrometers, and less than or equal to 0.3 micrometers.

According to different embodiments, the parameter R_(q) can be measuredalong one of the following sampling lengths: 10 micrometers to 20micrometers, 20 micrometers and 40 micrometers, 20 micrometers to 100micrometers, and 5 micrometers to 200 micrometers. When measuring theroughness of surface 36, the profile waviness that characterizes theshape of surface 36 should normally be subtracted before evaluatingR_(q). The definition of surface waviness is illustrated in FIG. 10which shows a surface profile (as it may be measured by a surfaceprofilometer, for example) and a centerline characterizing the overallshape of the object and which should be subtracted from the measuredprofile to determine parameter R_(q).

The waviness may be customarily subtracted by high-pass filtering with acut-off wavelength λ_(f) (see e.g., ASME B46.1-2009). The cut-offwavelength λ_(f) should be at least 5 to 10 times less than the samplinglength. On the other hand, the cut-off wavelength λ_(f) should be atleast several micrometers, for example, 2 to 5 micrometers or 5 to 20micrometers. The upper limits for cut-off wavelength λ_(f) may also bedefined by the size of light extraction feature 8. For example, cut-offwavelength λ_(f) may be set to at most the length L₈ or one-half of thelength L₈.

According to one embodiment, the parameter R_(q) of surface 11 should beless than 25 nanometers, more preferably less than 20 nanometers, evenmore preferably less than 15 nanometers and still even more preferablyless than or equal to 10 nanometers. The measurements of parameter R_(q)of surface 11 may be performed in a vicinity of light extraction feature8. The measurements should preferably utilize the same or similarsampling length as that used for measuring R_(q) of surface 36.According to one embodiment, the parameter R_(q) of surface 11 should beperformed along a direction that is perpendicular to a light input edge(e.g., perpendicular to edge surface 13).

According to one embodiment, the size of light extraction feature 8 mayrange from 10 micrometers to 200 micrometers in the longest dimension. Apreferred size range is 30-150 micrometers in the longest dimension. Aneven more preferred size range may be 30-80 micrometers in the longestdimension.

According to different embodiments, the volume of at least some ofprinted microdots forming individual light extraction feature 8 mayrange from 1,000 cubic micrometers to 10,000 cubic micrometers, from1,000 cubic micrometers to 100,000 cubic micrometers, from 10,000 cubicmicrometers to 100,000 cubic micrometers, from 20,000 cubic micrometersto 80,000 cubic micrometers, and from 30,000 cubic micrometers to 60,000cubic micrometers. According to one embodiment, the volume of at leastsome of individual printed microdots is about 4,000 cubic micrometers.According to one embodiment, the volume of each light extraction feature8 formed by one or more printed microdots is between 2,000 cubicmicrometers and 6,000 cubic micrometers.

The size of individual random surface microstructures 34 may range fromseveral nanometers to several micrometers. According to a preferredembodiment, microstructures 34 have sizes less than 0.5 microns.According to one embodiment, at least some microstructures 34 may havetop portions that can be approximated by spherical shapes having aradius of curvature between 50 nanometers to 200 nanometers.

According to one embodiment, each light extraction feature 8 includes agenerally opaque material (e.g., white pigment) but at such a lowthickness that the light extraction feature 8 is semi-opaque,non-absorbing and transmits at least a portion of light impinging on it.The term “non-absorbing”, in reference to opaque or semi-opaque layersof various materials used for making light extraction features 8 or 9,such as white pigment inks, for example, is directed to mean that therespective layer(s) do not perceptibly absorb light. For instance, anexemplary non-absorbing layer may be formed by an optically clear resinloaded with submicron TiO₂ particles at 10-20% concentration (byweight). The non-absorbing layer may have a relatively low thicknessbetween about 1 micrometer and 10 micrometers. Due to the low thicknessand the presence of submicron TiO₂ particles, the layer can beconfigured to partially reflect light and partially transmit lightwithout perceptible absorption. Accordingly, the sum of a totalreflected light energy E_(R) and a total transmitted light energy E_(T)can be equal (preferably within 1%-5% error) to a total light energy E₀incident onto the non-absorbing layer (E₀=E_(R)+E_(T)). According todifferent embodiments, the absorption within the semi-opaque material ata thickness equivalent to the average thickness of light extractionfeatures 8 and/or 9 is less than 5%, more preferably less than 3%, evenmore preferably less than 2% and still even more preferably less than1%.

As employed in the present specification and claims, the term “opacity”refers to the extent to which a surface, an object or a layer of amaterial impedes the transmission of light through it. For example, alayer or surface that completely prevents light passage is consideredcompletely opaque and having a 100% opacity. In contrast, a layer orsurface that transmits essentially all of the incident light isconsidered having a 0% opacity. Accordingly, a layer or surface thattransmits one-half of the incident light is considered having a 50%opacity.

For a partially opaque (semi-opaque) surface or layer which is alsoreflective and non-absorming, the opacity may be defined and measured asa ratio of the reflectance of the surface or layer against a lightabsorbing background surface to its reflectance against a highlyreflective background surface, at least according to some embodiments.The light absorbing background surface should preferably have <5%reflectance and even more preferably <3% reflectance. The highlyreflective backing surface should preferably have a hemisphericalreflectance of at least 89%, more preferably at least 95% and even morepreferably 98-99%. The highly reflective backing surface may bespecularly reflecting, diffusely reflecting or reflect light bothspecularly and diffusely.

In an alternative, according to some embodiments, the opacity may bemeasured using standard techniques, such as those described in ASTMD1746-15 or ASTM D589-97 documents and using a suitable standardizedopacity meter. When it is impossible to directly measure the opacity ofindividual semi-opaque light extraction features 8 or 9 (e.g., due totheir small size), the measurements may be performed indirectly using abroad-area layer of the same semi-opaque material deposited to atransparent substrate with a uniform thickness corresponding to aweighted average thickness of the material in the individual lightextraction features 8 and/or 9. For example, if a weighted averagethickness of the semi-opaque light extraction features is 8 micrometers(as measured, for instance, by a 3D microscope or a profilometer), atransparent substrate may be coated with a uniform 8-micrometer-thicklayer of the same material, and the opacity measurements of may beperformed for that layer in order to characterize the opacity of thelight extraction feature.

In a further alternative, the opacity may be expressed and measured interms of light attenuation by the material of light extraction features8 and/or 9. More specifically, the opacity may be defined by thefollowing expression: 100%(1−I_(T)/I₀), where I₀ is the intensity oflight incident onto the semi-opaque layer of light extraction featureand I_(T) is the intensity of light that is transmitted through thesemi-opaque layer.

According to one embodiment, the opacity and/or reflectivity of thesemi-opaque layers may be measured and/or compared in accordance withone or more other applicable standards, such as, for example, ISO 2814,ISO 6504, BS 3900-D4, BS 3900-D7, ASTM E97, ASTM E1347, ASTM D4214, ASTMD2805, and ASTM D589.

For the purpose of characterizing light extraction features 8 and/or 9,the opacity may provide a useful measure of the fraction of lightimpinging onto such light extraction features from the side of lightguiding sheet 10 that can be emitted from the other side of the lightextraction features (e.g., away from surface 11 when features 8 and/or 9are formed in that surface).

According to a preferred embodiment, referring to FIG. 9, the prescribedopacity of light extraction feature 8 is provided by scattering visiblelight in all directions in a tree-dimensional space using relativelytransparent particles 33 volumetrically distributed within the bulk ofbinder material 37. Preferred mechanisms for scattering light using suchparticles and without perceptible absorption include refraction,diffraction or a combination thereof.

According to one embodiment, light scattering particles 33 suspended inbinder material 37 may be formed by spherically shaped nanoparticles ofa relatively transparent material having a very high index of refraction(n≥2). Suitable exemplary materials particularly include but are notlimited toinorganic white pigments such as rutile or Anatase titaniumdioxide (n=2.5-2.8), antimony oxide (n=2.1-2.3), Zinc Oxide (n∞2), whitelead (basic lead carbonate, n=1.9-2.1), and lithopone (n=1.8).Alternatively, other inorganic materials or polymers having moderatelyhigh refractive indices (about 1.6 or greater) may also be used,including particularly magnesium silicate, baryte, calcium carbonate,calcium carbonate, polystyrene, or polycarbonate.

The sizes of particles 33 may be selected to maximize light scatteringin a particular range of wavelengths (e.g., visible wavelengths centeredaround 0.5 micrometers). Suitable sizes of particles 33 to maximizediffraction can be calculated, for example, using Mie theory of lightscattering.

According to one embodiment, an average size of particles 33 is about200 nanometers. According to one embodiment, an average size ofparticles 33 is about 250 nanometers. According to one embodiment, anaverage size of particles 33 is between 100 nanometers and 400nanometers. According to one embodiment, the size of particles 33 isbetween 150 nanometers and 350 nanometers.

Two or more different sizes (or size distributions) of particles 33 canalso be mixed within the material of the ink used to produce lightextraction features 8. Such mixing may be directed to maximize lightscattering at two or more different wavelength ranges. These differentwavelength ranges may be non-overlapping (e.g., 400-450 nm and 570-590nm) or overlapping (e.g., 400-550 nm and 450-600 nm).

The concentration of particles 33 in binder material 37 may vary in abroad range. According to preferred embodiment, the concentration ofparticles 33 (pigment loading) is between 5% and 35% by weight or volumein the respective ink material used to produce light extraction features8. According to one embodiment, the concentration is between 30% and 50%by weight. According to one embodiment, the concentration is between 5%and 25% by weight. According to one embodiment, the concentration isbetween 10% and 20% by weight.

According to some embodiments, light extraction features 8 and/or 9 aresubstantially non-absorbing and have the opacity of at least 10%, atleast 20%, at least 30%, at least 40% or at least 50%. According to someembodiments, the opacity of at least some individual light extractionfeatures 8 and/or 9 is less than 90%, approximately equal to or lessthan 80%, approximately equal to or less than 70%, or approximatelyequal to or less than 50%. According to one embodiment, the opacity ofindividual light extraction features 8 is between 30% and 70%. Accordingto one embodiment, the optical transmittance of non-absorbing,semi-opaque light extraction features 8 is one of the following: greaterthan 10%, greater than 20%, greater than 30%, greater than 40% and equalto or greater than 50%.

According to one embodiment, the opacity of the material forming lightextraction features 8 (e.g., white or fluorescent UV-curable ink) isbetween 40% and 70% when measured at a layer thickness of 10 micrometersor less. According to alternative embodiments, the opacity is between30% and 50% when measured at a layer thickness of 0.5 micrometers to 5micrometers, 1 micrometer to 2 micrometers, or 1 micrometer to 6micrometers.

A further useful measure of the opacity of light extraction features 8and/or 9 and the ability of light guiding sheet 10 to emit light fromboth sides (e.g., from both opposing surfaces 11 and 12 is a luminanceratio between the respective opposing surfaces. For example, let'sconsider an embodiments of wide-area illumination system 900 in whichlight extraction features 8 are formed by printing microscopic opaque orsemi-opaque white-ink dots on surface 11. In an illustrative,non-limiting example, when light guiding sheet 10 is patterned withlight extraction features 8 and illuminated from one or two edges byLEDs 2, an average measured surface brightness of surface 11 may beabout 4,500 cd/m² while an average measured surface luminance of surface12 may be about 1,500 cd/m², thus giving a 3 to 1 ratio. In other words,light guiding sheet 10 will emit about 75% of light from surface 11 andabout 25% from opposing surface 12. For the purpose of determining theopacity of light extraction features 8 and/or 9, the surface luminanceshould preferably be measured from a perpendicular direction withrespect to surfaces 11 and/or 12.

According to one embodiment, the opacity and light scattering propertiesof light extraction features 8 are configured such each of surfaces 11and 12 outputs 30% to 70% of the total light emitted by sheet 10 throughsurfaces 11 and 12. In other words, surface 11 may be configured tooutput no less than 30% of the total light energy emitted from sheet 10as a result of light extraction by light extraction features 8, and, atthe same time, surface 12 may also be configured to output no less than30% of the total light energy emitted from sheet 10 as a result of suchlight extraction. According to different embodiments, the approximatelyproportions between light output from surfaces 11 and 12 may be 30%:70%,35%:65%, 40%:60%, 45%:55%, 50%:50%, 55%:45%, 60%:40%, 65%:35%, or70%:30%.

According to some embodiments, the opacity of light extraction features8 may be selected such that the ratios between the measured luminance ofsurfaces 11 and 12 are about 1:1 (about equal surface luminance ofsurfaces 11 and 12), 1.1:1, 1.2:1, 1.3:1, 1.5:1, 2:1, 3:1, 3.5:1, 4:1,4.5:1, and 5:1. According to some embodiments, the opacity of lightextraction features 8 may be selected such that the ratios between themeasured total output from surface 11 and surface 12 are about 1:1(about equal amounts of light are emitted from both surfaces), 1.1:1,1.2:1, 1.3:1, 1.5:1, 2:1, 3:1, 3.5:1, 4:1, 4.5:1, and 5:1. Suchconfigurations with two-sided light output may be advantageouslyselected, for example, for applications that will benefit fromdirect/indirect illumination. Furthermore, configurations of systems 900with semi-opaque light extraction features 8 may be advantageouslyselected for applications that will benefit from the smallest sizeand/or thickness of the light extraction features and require that thelight extraction pattern is virtually invisible to a naked eye atrelatively close viewing distances.

It is noted that the use of semi-opaque light extraction features is notlimited to the cases of two-sided illumination. FIG. 9 furtherillustrates the operation of system 900 when semi-opaque, substantiallynon-absorbing light extraction feature 8 is used together with areflective back sheet. Reflective sheet 41 should preferably have aspecular or diffuse hemispherical reflectance of at least 85-95%.

In operation, a light ray 301 exemplifies light that isextracted-decoupled from light guiding sheet 10 and distributed/emittedfrom wide-area illumination system 900 using a multi-stage process. Ray301 initially propagates in light guiding sheet 10 in a waveguide mode.Subsequently, ray 301 enters semi-opaque light extraction feature 8 andencounters one of light scattering particles 33 which splits the energyof ray 301 into two distinct portions propagating toward differentdirections. A first portion of ray 301 is forward-scattered by particle33, forming a ray segment 303. A second portion of ray 301 isforward-scattered by the same particle 33, forming a ray segment 302.

Ray segment 302 is further forward-scattered by an adjacent particle 33and is directed towards surface 12 (not shown) at an angle below thecritical TIR angle characterizing surfaces 11 and 12, so it can befinally extracted from sheet 10 and emitted from surface 12. Ray segment303 is further emitted from surface 36 and reflected back towards sheet10 by reflective sheet 41. Ray segment 303 re-enters the body of lightextraction feature 8 and is further propagated back into sheet 10,undergoing additional interactions with light scattering particles 33.Similarly, ray segment 303 enters onto sheet 10 at a below-TIR angle,allowing for decoupling ray segment 303 from light guide 800.

According to an aspect, ray segment 302 exemplifies light that isback-scattered towards surface 11 and that eventually re-enters backinto light guiding sheet 10 and can be emitted from opposing surface 12(not shown), contributing to the total emission from system 900. Raysegment 303 exemplifies light that is initially forward-scattered awayfrom surface 11 and that contributes to the emission from surface 11.Accordingly, in the absence of sheet 41, the total light emission fromsheet 10 would be distributed between surfaces 11 and 12 according to acertain ratio. This ratio can be determined and controlled, at least inpart, by the opacity of light extraction feature(s) 8. In theillustrated example, however, ray segment 303 is intercepted by highlyreflective sheet 41 and reflected back towards light guiding sheet 10.Ray segment 303 further passes through the semi-opaque layer of lightextraction feature 8 for the second time, undergoing some additionalscattering, so that it can be ultimately emitted from opposite surface12 of light guiding sheet 10.

It is noted that individual light rays being extracted by semi-opaquelight extraction features 8 may undergo multiple back-scattering andforward-scattering reflections within the light-scattering layer, e.g.,as schematically illustrated by various segments of ray 301 in FIG. 9.Additionally, the extracted light rays may further undergo multiplepassages through the same light extraction feature 8 or even differentlight extraction features 8. The number of interactions of the extractedlight ray with light-scattering particles 33 may depend on the size ofsuch particles, their distribution density within the semi-opaque layer,height H₈, spacing between light extraction features 8, and otherfactors. The thickness, opacity, light transmittance and the size ofparticles 33 may be selected such that the light scattering is maximized(for example, by maximizing light diffraction by particles 33 andproviding for at least double or multiple passage of light through thematerial of light extraction features 8) while light absorption withinthe body of light extraction features 8 is minimized.

According to one embodiment, the light scattering provided bysemi-opaque, semi-transmissive light extraction features 8 with orwithout the aid of reflective sheet 41 is such that light emitted fromlight guiding sheet 10 has a Lambertian or quasi-Lambertian angulardistribution. In other words, the luminous intensity observed fromsurface 12 of light guiding sheet 10 can be approximately proportionalto the cosine of the observation angle (an angle between the observationdirection and a surface normal). According to one embodiment, theangular emission distribution can be approximated by a Lambertian cosinelaw at least in the YZ plane. According to one embodiment, the angularemission distribution can be approximated by a Lambertian cosine law atleast in the XZ plane. According to one embodiment, the angular emissiondistribution can be approximated by a Lambertian cosine law in both XZand YZ planes.

According to some embodiments, light guide illumination system 900 ofFIG. 9 may be used without reflector 41 and can still be configured forproviding a Lambertian or quasi-Lambertian angular distribution of lightemission, e.g., from surface 11, surface 12 or both surfaces 11 and 12.According to some embodiments, light extraction features 8 may beconfigured to provide a “bat-wing” angular distribution of lightemission from either one or both surfaces 11 and 12.

The average thickness of the semi-opaque layer of each micro-printedlight extraction feature 8 may be selected to provide a prescribed ratiobetween forward-scattering and back-scattering. In order to maintain aminimum prescribed level of light transmittance for light extractionfeatures 8, height H₈ (maximum thickness) or the average thickness ofthe respective semi-opaque layer may be limited to certain values. Insome embodiments, height H₈ or the average thickness may be equal to orless than 15 micrometers, equal to or less than 10 micrometers, lessthan 8 micrometers, equal to or less than 6 micrometers, equal to orless than 5, equal to or less than 3 micrometers, equal to or less than2, between 1 and 2 micrometers, between 1 and 1.5 micrometers, about 1micrometer, or between 0.5 and 1 micrometers. At the same time, thecharacteristic size (e.g., dimension L₈ or diameter d) of features 8 mayrange from 30 micrometers to 150-200 micrometers and more preferablyfrom 30 micrometers to 80 micrometers. According to differentembodiments, a prevalent size or diameter of light extraction features 8is about 50 micrometers, 60 micrometers, 70 micrometers, 80 micrometers,90 micrometers, or 100 micrometers. According to one embodiment, theprevalent size or diameter of light extraction features 8 is between 50micrometers and 80 micrometers. According to one embodiment, the maximumthickness of the semi-opaque layer of light extraction feature 8 may begreater or equal to 1 micrometer and less than or equal to 10micrometers. According to one embodiment, the maximum thickness may havevalues from 1 micrometer to 8 micrometers, from 1 micrometer to 6, from2 micrometer to 6, or from 2 micrometer to 4 micrometers.

According to at least some embodiments, the ratios between L₈ and H₈ maybe as low as 3 or as high as 100 or so. According to some embodiments,the ratio between L₈ and H₈ may be between 5 and 100, between 10 and200, between 10 and 100, between 20 and 100, between 40 and 100, andbetween 40 and 80.

According to one embodiment, highly reflective sheet 41 is configured toreflect primarily by a specular reflection (causing angles of reflectionbeing equal to the angles of incidence). According to one embodiment,highly reflective sheet 41 is configured to reflect primarily in adiffuse regime and thus provide additional scattering to the extractedlight compared to scattering/deflecting light using light extractionfeatures 8 only.

The opacity of individual light extraction features 8 and the density ofsuch light extraction features on surface 11 and/or 12 may be configuredto control the opacity of light guiding sheet 10 in different areas. Inan extreme exemplary case, the density of light extraction features 8can be made very high, such that there is practically no spacing betweenindividual light extraction features 8 (e.g., separation distances SEDare about zero or less than zero) and that light guiding sheet 10 or itsportions are substantially opaque. In a further example, by selecting aneven higher packing density (with separation distances SED beingsignificantly less than zero but greater than −d/2) the opacity of lightguiding sheet 10 may be made similar to that of individual lightextraction features 8.

According to one embodiment, light extraction features 8 may be formedby stretchable inks. In different implementations, the fully cured inkmaterial should allow for its reversible stretching without cracking inthe elastic or plastic mode by at least 10%, 30%, 50%, 10%, 150%, or200% elongation.

EXAMPLE 1

FIG. 11 shows an exemplary measured dependence of the opacity of lightguiding sheet 10 on spacing between light extraction features 8. In theillustrated example, light guiding sheet 10 was formed by an opticallyclear acrylic sheet having a 0.75-mm thickness. Light extractionfeatures 8 were formed by depositing microdrops of UV-curable white inkon a surface of the acrylic sheet using a commercial flatbed UV printerwith instant curing of the respective micro-droplets.

The individual printed light extraction features 8 had sizes around120-130 micrometers along the longest dimensions and an average totalthickness of about 6-8 micrometers. In each sample pattern, lightextraction features 8 were arranged in a two-dimensional array having afixed spacing (pitch) in the X and Y dimensions (fixed spacing distanceSPD).

The patterns having microdrop spacing below 100 micrometers completelycovered the surface with the ink (reaching a 100% fill factor) due tooverlapping of adjacent microdrops, which corresponded to the case ofnear-zero or slightly negative separation distances SED. At a microdroppitch of 84 micrometers, the pattern produced a continuous layer ofwhite ink with a measured average thickness of 7-8 micrometers(substantially overlapping microdrops with separation distances SEDbeing significantly less than zero). At an even lower microdrop pitch,the thickness of the resulting layer was measured at about 30micrometers.

Referring further to FIG. 11, relatively sparsely populated, discretelight extraction features 8 (e.g., spaced by distances SPD of 0.3 mm to0.6 mm) produced fairly low levels of opacity for light guiding sheet10. On the other hand, densely populated light extraction features 8(e.g., spaced by distances SPD of 0.1 mm and below) produced moderate torelatively high levels of opacity for the light guiding sheet.

At a pitch (or spacing distance SDD) of about 42 micrometers, theopacity of sheet 10 reached a maximum at about 66%. Accordingly, at thisopacity level, light guiding sheet diffusely reflected about two thirdsof the incident light and diffusely transmitted around one third of theincident light. The absorption within the layer of white ink was foundto be less than 3-5% (less than or comparable to the measurementerrors).

End of Example 1

The graph of FIG. 11 can be approximated by a polynomial function andinterpolated or extrapolated to calculate areal pattern coverage toachieve a prescribed level of opacity. In view of such results, it isnoted that, according to at least some embodiments, the opacity ofsemi-opaque light extraction features 8 may also be approximatelydetermined by measuring the opacity of light guiding sheet 8,particularly in the areas of relatively high density of light extractionfeatures 8 and extrapolating the results to the 100% coverage of thesurface with the respective semi-opaque layer, at the appropriate layerthickness.

The term “surface coverage” may be defined as the ratio between acumulative area of light extraction features 8 within a sampling regionand the total area of the sampling region. The area of the samplingregion should be at least 100 times greater than the average or typicalarea of individual light extraction features 8. According to oneembodiment, light guiding sheet includes at least one region having asurface coverage of at least one of the following: 10% or more, 20% ormore, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more,80% or more, and 90% or more. According to some embodiments, spacingareas between light extraction features 8 cumulatively occupy less than50% of the total light guide area within a sampling region, less than30%, less than 20%, or less than 10%. According to one embodiment, lightguiding sheet 10 includes at least one region having the surfacecoverage of at least one of the following: less than 10%, less than 7%,less than 5%, and less than 2%.

Light-scattering particles 33 may be randomly varied in size within asingle light extraction feature 8. Furthermore, any individual lightextraction feature 8 may include a number of randomly formedagglomerates 35 that represent localized regions of increased density oflight-scattering particles compared to surrounding areas.

According to one embodiment, at least some of light scattering particles33 include one or more fluorescent materials. Fluorescent materials mayalso be mixed with non-fluorescent (e.g., color filtering or lightscattering) materials in various proportions, e.g., 10%:90%, 50%:50%, or90%:10%.

Light guiding sheet 10 may be configured such that a first lightextraction pattern of light extraction features 8 formed in surface 11and a second light extraction pattern of light extraction features 9 isformed in surface 12. The second pattern of light extraction features 9may cover the same area of sheet 10 as the first pattern of lightextraction features 8. The first and second patterns may superimpose onone another in terms of X and Y coordinates of the respective individuallight extraction features 8 and 9. The second pattern may also be arotated and/or translated copy of the first pattern. According to oneembodiment, the first and second patterns may have different pitch ordifferent surface distributions of the respective light extractionfeatures 8 and 9, for example, for the purpose of reducing the chance ofthe so-called moire effect when the light emitting surface of system 900is observed by a viewer.

The shapes, sizes, distribution densities and orientations of lightextraction features 8 and/or 9 may vary in a broad range. According toone embodiment, light extraction features 8 and/or 9 may have randomshapes, sizes and/or orientations across respective surfaces 11 and/or12. This may also be useful, for example, for reducing the conspicuityof the patterns and providing a perceptibly uniform light output fromillumination system 900.

FIG. 12 schematically shows various exemplary shapes, densities andorientations of various light extraction features that can be formed inor on surfaces 11 and 12 of light guiding sheet 10. Light extractionfeatures 701, 702, 705, and 706 exemplify various-shape microdots ofwhite UV-curable ink printed on surface 11.

More particularly, light extraction features 701 exemplify regular orquasi-regular shapes of the microdots which may be have some small-scaleshape irregularities. Light extraction features 702 exemplify printedmicrodots having irregular or highly irregular shapes. Light extractionfeature 706 exemplifies regular or quasi-regular shapes of microdotsthat have a somewhat fussy outline. Light extraction features 705exemplify printed microdots that have much smaller sizes compared tolarger light extraction features 701, 702, and 706. The smaller-sizedlight extraction features 705 are provided in spaces between thelarge-sized light extraction features and can have a generally differentdistribution pattern than the larger light extraction features.

According to one embodiment, light extraction features 701, 702, and 706may have volumes between 30,000 and 100,000 cubic micrometers while eachof light extraction features 705 can have a volume below 10,000 cubicmicrometers, below 5,000 cubic micrometers, between 1,000 cubicmicrometers and 5,000 cubic micrometers, or less than 1,000 cubicmicrometers.

Referring to FIG. 10, a fully cured micro-drop of UV-curable ink may becharacterized by a contact angle which is defined, by an analogy fromthe wettability characterization of liquid drops on a solid substrate,as the angle between a curved surface of the micro-drop and thesubstrate surface (e.g., surface 11) where the respective surfaces meet.When the micro-drop has a rough, microstructured surface, the surfaceroughness should subtracted from the surface profile before measuringthe contact angle. For example, a suitable waviness profile (see FIG. 10and the above discussion in reference to FIG. 10) may be used as arepresentation of the shape of the micro-drop and the contact angle maybe measured using such waviness profile.

According to one embodiment, at least some of light extraction features8 and/or 9 are formed by UV-cured micro-drops having a contact angle ofabout 10 degrees. According to one embodiment, the contact angle isabout 5 degrees. According to one embodiment, the contact angle is about15 degrees. According to one embodiment, the contact angle is about 20degrees. According to one embodiment, the contact angle is between 2degrees and 5 degrees. According to one embodiment, the contact angle isgreater or equal to 1 degree and less than or equal to 2 degrees.According to one embodiment, the contact angle is less than 1 degree.According to one embodiment, the contact angle is greater than 0.1degree and less than 1 degree. According to one embodiment, the contactangle is between 5 degrees and 25 degrees. According to one embodiment,the contact angle is between 5 degrees and 20 degrees. According to oneembodiment, the contact angle is between 10 degrees and 20 degrees.According to one embodiment, the contact angle is less than 5 degrees.According to one embodiment, the contact angle is greater than 25degrees.

It is noted that the contact angle of liquid (uncured) micro-drops, maybe about the same but may also be generally different from that of thefully-cured, solidified micro-drops. Furthermore, at least someembodiments may include printed light extraction features that areformed by solvent-based micro-drops in which case the contact angles ofthe liquid and fully-cured micro-drops may differ significantly due tothe evaporation of the solvent during the curing process. According toone embodiment, light extraction features 8 are formed by microdrops ofeither radiation-curable or solvent-based materials which are depositedto the respective surfaces using known methods other than printing. Forexample, such materials may be deposited to surface 11 in the form ofliquid microdrops using tightly controlled pressure spraying, ultrasonicspraying, or a combination thereof.

According to one embodiment, a method of making wide-area illuminationsystem 900 may include a step of generating a two-dimensional pattern ofdiscrete locations (e.g., X, Y coordinates) of light extraction features8 where spacing distances between the discrete locations graduallyincrease or decrease with a distance from an edge of the pattern. Thetwo-dimensional pattern may be generated, for example, usingcomputer-based modeling, such as raytracing. The optical modeling may beconstrained to provide a substantially uniform expected light outputfrom light guide 800.

The method of making wide-area illumination system 900 may furtherinclude a step of converting the pattern of discrete locations to acomputer readable raster bitmap having a two-dimensional array ofpixels. Suitable examples of computer readable raster bitmaps that canbe used to store coordinates of light extraction features 8 include, butare not limited to bitmap image file format (e.g., the BMP file format),device independent bitmap (e.g., DIB), tagged image file format (TIFF),portable document format (PDF), JPEG file format, portable networkgraphics (PNG) file format, graphics interchange format (GIF), and thelike. According to one embodiment, it may be preferred that the rasterbitmap is stored in a bitonal (e.g., black-and-white) form. For example,in a black-and-white bitmap, pixels corresponding to the locations lightextraction features 8 may be white and pixels corresponding to spacingareas can be black, or vice versa.

The method of making wide-area illumination system 900 may furtherinclude steps of providing a UV printing machine (a UV printer),providing a light guide substrate (e.g., highly transparent acrylicsheet), loading UV-curable inks of a preselected color (e.g. whiteUV-curable inks) into the UV printing machine, loading the bitmapcontaining information on locations of light extraction features 8 intoa software that is used to control the UV printing machine, printing thebitmap on the light guide substrate using the UV printing machine, andcuring the droplets deposited onto the surface of the light guidesubstrate (e.g., using UV light from UV LED sources). The curing processcan be performed simultaneously with the printing process or as aseparate post-printing step.

The method of making wide-area illumination system 900 may furtherinclude various software and/or printer configuration steps, such as,for example, specifying the print resolution, target print areadimensions, and target size of the printed dots/microdots. According toone embodiment, printing can be performed at a resolution of 600 dotsper inch (DPI). According to one embodiment, printing can be performedat a resolution of 1200 DPI, 1800 DPI, 2400 DPI or a higher DPI.According to one embodiment, printing can be performed at a resolutionof 300 DPI or lower. The DPI resolution of the bitmap associated withthe print may be selected to match that of the printing resolution. Thesize of printed dots may be specified directly, e.g., via appropriatesoftware settings, or indirectly, e.g., by selecting appropriateprinting regimes that result in a prescribed size of the individualmicrodots.

The formation of individual microdrops within the printer may ordinarilybe performed using a drop-on-demand head that electrically actuates apiezoelectric crystal to produce ink drops of a prescribed size inresponse to voltage pulses. According to one embodiment, it can bepreferred that each actuation of the piezoelectric crystal results indepositing a single drop of ink to each prescribed location of the lightextraction features 8 (e.g., the locations corresponding to a “white”pixels on a “black” background in the raster bitmap).

According to one embodiment, the method of making wide-area illuminationsystem 900 includes a continuous or at least intermittent recirculationof the ink during the printing process. Including this step or processcan be especially important for white ink compositions that includeheavy particles suspended in a much lighter liquid material (e.g.,light-scattering nanoparticles of TiO₂ suspended in a clear bindermaterial). Without the recirculation, the heavy particles may causevarious issues such as, for example, ink sedimentation in the fluidpaths, clogging the jetting nozzles, and creating non-uniformities ofvolumetric particle loading in the ink. This, in turn, may impact inkdischarge and severely affect or even prevent the formation ofsingle-droplet microdots (individual light extraction features 8) of theprescribed size and volume. The continuous recirculation can beperformed continuously or intermittently by agitating the dispersion orsuspension of the heavy particles within a closed-path independent fluidcircuit using a recirculation pump. The closed-path fluid recirculationcircuit can be located anywhere along the ink supply line, e.g, betweena reservoir containing the ink and the printing head. According to oneembodiment, the closed-path fluid recirculation circuit may also bebuilt into the printhead.

According to one embodiment of the method of making wide-areaillumination system 900, depositing of different microdots within arelative small area (e.g., within a band hawing a width from severalmillimeters to several centimeters) may be performed using two or moreconsecutive printing passes of the print head over the same area.According to one embodiment, the method of making wide-area illuminationsystem 900 includes setting a prescribed number of individual drops tobe deposited to the same location. This regime can be advantageouslyselected for building the prescribed thickness of light extractionfeatures using a finite pre-defined number of relatively smallmicrodots.

According to one embodiment, the method of making wide-area illuminationsystem 900 may further include bending the light guide substrate to acurved shape and making it operable for distributing light while beingin a bent or curved state. According to one embodiment, the lightguiding substrate may be sandwiched between a flexible back-sheetreflector and a flexible, film-thickness transmissive diffuser sheet(e.g., for a single-sided diffuse emission), both of which can be bentand flexed together with the light guiding substrate. The back-sheetreflector may specular or diffuse and may conventionally have a filmthickness for enhanced flexibility. According to one embodiment, theback-sheet reflector may be replaced with another flexible,film-thickness transmissive diffuser sheet (e.g., for a two-sideddiffuse emission).

FIG. 13 schematically illustrates an embodiment of wide-areaillumination system 900 in a front light configuration withsubstantially single-sided emission. Referring to FIG. 13, there isprovided an image print 755 having a full-color viewable surface 757.Light extraction features 8 are configured for illuminating surface 757in a reflective mode of operation. Surface 11 is facing away from imageprint 755 and configured as a front viewing surface for image print 755.Surface 12 is facing towards image print 755 and configured forilluminating the image print.

It is noted the front light implementation of illumination system 900 isnot limited to illuminating an image print and may be used forilluminating space in front of surface 12 or illuminating any types ofobjects or surfaces. In different embodiments, image print 755 may bereplaced with a textured surface, graphics, indicia, logo, sign,letters, background surface (white, monochrome or colored), fabric,conventional image, stereoscopic image, photograph, LCD display, logo orpattern. According to one embodiment, a three-dimensional object orsurface may be used in place of image print 755. According to oneembodiment, a layer of fluorescent material may be used in place of orin conjunction with image print 755.

Each light extraction feature 8 of FIG. 13 is formed by an innerreflective layer 777 and an outer opaque layer 778. The inner layer 777is formed by a semi-opaque, highly reflective material which includestransparent binder material 37 and high-refractive-index lightscattering particles 33 distributed throughout the volume of bindermaterial 37. Outer layer 778 is formed by a highly opaque materialhaving a sufficient thickness to block at least a substantial portion oflight that may be escaping from inner layer 777. According to oneembodiment, the highly opaque material may include a light absorbingmaterial. The opacity of outer layer 778 is preferably greater than 75%,more preferably greater than 80%, even more preferably greater than 85%,even more preferably greater than 90%, even more preferably greater than95%, and still even more preferably greater than 97%. According to oneembodiment, the opacity of layer 778 is substantially 100%. Suitablematerials for light absorbing outer layer 778 include, for example,black inks including a carbon black pigment.

The inner light-reflecting layer 777 is facing towards image print 755and the outer absorptive layer 778 is facing towards a viewer 660. Itmay be appreciated that, by utilizing the inner layer which is highlyreflective, light scattering and semi-opaque (also preferably beingbright-white in color) and the outer layer which is light-absorbing andhighly opaque, illumination system 900 may be configured to suppress oreven completely eliminate glare associated with a light emission fromlight extraction features 8 towards viewer 660. Accordingly, thisconfiguration may be advantageously used as a front light forilluminating image print 755 with a high contrast and without unwantedglare.

For example, it can be shown that, if the opacity of the inner layer 777is 70% and the opacity of the outer layer 778 is 90%, the combinedopacity can be about 97% (resulting in only 3% of light being emittedtowards viewer 660). According to different embodiments, the total(combined) opacity of layered light extraction features 8 is one of thefollowing: 90%, 92%, 94%, 96%, 98%, and 99%.

Opaque layer 778 of light absorbing material is conformably coating thesurface of the semi-opaque inner layer 777 such that minimum orvirtually no light is emitted from the respective light extractionfeature 8 directly towards viewer 660, even when illumination system 900is fully lit by LEDs 2 coupled the respective edges of light guidingsheet 10. According to one embodiment, light extraction features 8 areconfigured to be essentially invisible to viewer 660 when illuminationsystem 900 is in the “on” (illuminated) state. This can be achieved, forexample, by making the size of individual features 8 sufficiently smalland by making the opacity of respective outer layers sufficiently high.According to one embodiment, light extraction features 8 are also madeessentially invisible to a viewer 900 when illumination system 900 is inthe “off” state (i.e., when not illuminated by LEDs 2). According to oneembodiment, light extraction feature 9 are also essentially invisible toa viewer 900 when illumination system 900 is either in the “off” or “on”state. According to one embodiment, light extraction features 8 are madeessentially invisible to viewer 660 at a normal viewing distance whenillumination system 900 is a non-illuminated state but visible at thesame distance when system 900 is in an illuminated state.

It is preferred that opaque layer 778 is formed on top of thethree-dimensional structure of semi-opaque inner layer 777 as aconformable coating. The term “conformable” with respect to a coatingrefers to a layer that generally conforms in shape to the underlyingthree-dimensional surface, layer and/or structure, such as a curvedouter surface of the inner layer 777 of light extraction feature 8 ofFIG. 13, for example.

Outer opaque layer 778 preferably has the dimensions and shapeapproximating those of the inner semi-opaque layer 777 so as tocompletely cover inner layer 777. According to different embodiments,outer layer 778 covers at least 80%, at least 90%, or at least 95% ofthe surface of inner layer 777. According to one embodiment, outer layer778 covers 100% of the surface of inner layer 777.

According to one embodiment, outer opaque layer 778 has slightly largerdimensions or slightly larger area (e.g., by 5-10%) than those of innerlayer 777, for example, to ensure that no perceptible amount of straylight can escape from light extraction feature 8 towards the viewer.According to one embodiment, at least outermost portions of outer layer778 of light absorbing material are disposed in contact with surface 11such that outer layer 778 completely encapsulates inner layer 777. Onthe other hand, the size of layer 778 should be limited to reduceunwanted extraction and absorption of light propagating in light guide800. According to different embodiments, it is preferred that a widthWCA of optical contact area of outer layer 778 with surface 11 on eitherside of inner layer 777 is less than 30%, less than 20% or less than 10%of the diameter of the surface structure formed by inner layer 777.

The operation of wide-area illumination system 900 in aglare-suppressing front-light configuration is further illustrated bythe example of a path of a light ray 341. Ray 341 propagating in lightguiding sheet 10 in a waveguide mode enters binder material 37 where itoptically interacts with one or more light scattering particles 33. Alight ray segment 343 exemplifies a portion of light ray 341 that isforward-scattered upon such interaction. A light ray segment 342exemplifies a portion of light ray 341 that is back-scattered (diffuselyreflected towards sheet 10).

Ray segment 343 propagates further towards the outer light absorbinglayer 778 where it is substantially absorbed. In contrast, ray segment342 propagates back to light guiding sheet 10 where it overcomes TIR atsurface 12 and illuminates surface 757 of image print 755. Surface 757further reflects and scatters light exemplified by ray segment 342 anddirects the reflected light towards viewer 660. Accordingly, viewer 660can see image print 755 being illuminated with high contrast and withoutglare that could be otherwise caused by stray light emanated from lightextraction feature 8 in the absence of opaque layer 778.

To prevent the visibility of individual light extraction features 8 atrelatively short viewing distances (50 cm or less), the light extractionfeatures should preferably be smaller than 150-200 micrometers, morepreferably smaller than 100-150 micrometers, even more preferablysmaller than 100 micrometers, and still even more preferably smallerthan 80 micrometers. In some instances, however, e.g., when the viewingdistances are one meter, several meters or more, the size of lightextraction features 8 may be selected to be 300 micrometers or more, 0.5millimeter or more, 1 millimeter or more, and up to several millimetersor more.

The opacity of the material forming outer light absorbing layer 778should preferably be significantly greater than the opacity of thematerial forming the inner layer of light extraction features 8.According to one embodiment, light absorbing layer 778 may include acolor pigment. It may also include highly reflective (e.g., metallic)particles in concentrations sufficient to provide enhanced opacity forthe layer. According to one embodiment, opaque outer layer 778 may havereflective properties and provide enhanced opacity due to reflectionrather than absorption. According to one embodiment, opaque outer layer778 maybe replaced with a highly reflective layer that likewise providesenhanced opacity. In this case, light rays striking the respectiveopaque layer (e.g., ray segment 343 of FIG. 13) may be recycled byreflecting such rays back towards light guiding sheet 10 and image print755. This configuration may be advantageously selected to enhance systemefficiency and the apparent brightness of illuminated image print 755compared to the case of employing light-absorbing outer layer 778.

Light extraction features 8 and/or 9 may include other layers havingdifferent functions. According to one embodiment, a layer of highlytransparent material 779 may be provided between particle-loaded bindermaterial 37 and surface 11, as further illustrated in FIG. 13.Transparent layer 779 may be provided, for example, for enhancing lightextraction from light guiding sheet 10 or to promote adhesion of innerlayer 777 to surface 11. Transparent layer 779 should preferably have arefractive index that is about the same or greater than that of lightguiding sheet 10. Transparent layer 779 may also include a material thathas enhanced adhesion to surface 11 compared to the material of layer777. According to one embodiment, the adhesion of the material oftransparent layer 779 to surface 11 is greater than the adhesion of thematerial of binder material 37 to surface 11.

According to one embodiment of a method of making wide-area illuminationsystem 900 in a front light configuration, layered light extractionfeatures 8 may be formed by a sequential deposition or reflective andopaque materials to the same discrete X and Y locations of surface 11.For example, light extraction features 8 may be formed by initiallyprinting a suitable pattern of microdots of a UV-curable white ink onsurface 11 in a first pass followed by printing the same pattern ofmicrodots of a UV-curable black ink on top of the white microdots in asecond pass. According to one embodiment, such overprinting may beperformed in succession without repositioning the substrate (lightguiding sheet 10) between the first and second passes. This can be done,for example, using a commercial inkjet printer that is capable ofprinting with different types/colors of ink simultaneously. Similarly,triple-layer light-extraction features 8 (bottom of FIG. 13) may beprinted sequentially in three passes of a print head, without substraterepositioning between the printing passes.

FIG. 14 schematically depicts an embodiment of wide-area illuminationsystem 900 in a front-light configuration which is similar to that ofFIG. 13 except that light absorbing outer layers 778 conformably coatingan inner layer of light extraction features 8 are replaced with detachedlight blocking features 782 disposed in registration with reflectivelayers 777. Light blocking features 782 are formed on a thin transparentsubstrate sheet 781 covering surface 11.

Light blocking features 782 are distributed over substrate sheet 781according to the same two dimensional pattern as light extractionfeatures 8. Furthermore, the patterns of light blocking features 782 andlight extraction features 8 are precisely aligned relatively to eachother so that each light blocking feature 782 provides a discrete opaquecover for the respective light extraction feature 8.

Substrate sheet 781 may be advantageously separated from light guidingsheet 10 by a thin spacing layer 784 to accommodate the thickness oflight extraction features 8 and to provide an air gap sufficient tomaintain TIR in sheet 10. Substrate sheet 781 may conventionally have afilm thickness which can be much less than the thickness of lightguiding sheet 10, e.g., by at least 2 times, 3 times, 5 times, 10 timesor more.

Each light blocking feature 782 is formed by an opaque material whichcan be light absorptive or reflective. According to one embodiment,light blocking features 782 may be deposited to a surface of substratesheet 781 that is facing light guiding sheet 10. According to oneembodiment, light blocking features 782 are deposited to a surface ofsubstrate sheet 781 that is facing away from light guiding sheet 10.Each light blocking feature 782 should be disposed in registration withthe respective light extraction feature 8 and should preferably coverthe entire area of light extraction feature 8 from the viewer.

According to one embodiment, the size of each light blocking feature 782is at least the same or larger than the size of the respective lightextraction feature 8.

According to different embodiments, the size of light blocking features782 may be greater than the size of the respective light extractionfeatures 8 by at least 10%, 20%, 30%, 50%, or 100%. According todifferent embodiments, the area of light blocking features 782 may begreater than the area of the respective light extraction features 8 byat least 10%, 20%, 30%, 50%, 2 times, 3 times and 4 times.

According to one embodiment of a method of making wide-area illuminationsystem 900 in a front light configuration, the method includes a step ofdepositing a predetermined pattern of microdots of white or fluorescentUV-curable ink (light extraction features 8) to a surface of a lightguiding substrate (e.g., surface 11 of light guiding sheet 10), a stepof covering the surface of the light guiding substrate with a thintransparent substrate (substrate sheet 781) and a step of depositing thesame pattern of microdots of a highly opaque (preferably black orreflective) ink to a surface of the thin transparent substrate (e.g., toform light blocking features 782).

In operation, the embodiment of wide-area illumination system 900 ofFIG. 14 is similar to that of FIG. 13 except that forward-scatteredlight (as exemplified by ray segment 343) can escape from lightextraction features 8 and can propagate a relatively short distance awayfrom surface 11 and towards viewer 660 before it is blocked from furtherpropagation towards viewer 660 by light blocking features 782. Dependingon the type of the opaque material used for making light blockingfeatures 782 (e.g., absorbing or reflective), ray segment 343 may beabsorbed or reflected and recycled, similarly to some embodimentsdescribed above in reference to FIG. 13.

According to an aspect, transparent substrate sheet 781 and a pattern oflight blocking features 782 form an opaque mask or overlay thatselectively blocks light emitted from areas of light guiding sheet 10corresponding to light extraction features 8. At the same time, thetransparent spacing areas between light blocking features 782 allow fora general unimpeded transverse light passage from light guide 800 toviewer 660 and thus allow for a generally unimpeded viewing of imageprint 755. It may be appreciated that, with a proper alignment of themask or overlay and with the proper sizing of light blocking features782, the glare associated with individual semi-opaque light extractionfeatures 8 may be suppressed or even eliminated such that the apparentcontrast for illuminated image print 755 may be significantly enhancedcompared to the case where no such mask or overlay is used.

FIG. 15 schematically shows an embodiment of wide-area light guideillumination system 900 that has three planar light guides stacked onone another along the Z axis. Light guiding sheet 10 represents a firstplanar light guide, a light guiding sheet 20 represents a second planarlight guide, and a light guiding sheet 30 represents a third planarlight guide.

According to one embodiment, sheets 20 and 30 may be made from the samematerial as sheet 10. Sheets 20 and 30 may also have similar oridentical dimensions and structure as sheet 10. According to oneembodiment, each of the sheets 10, 20 and 30 may differ from the othertwo sheets in one or more of the following: material, structure,composition, thickness, length and/or width dimensions, color, surfacetexture, and light extraction patterns.

Sheets 10, 20 and 30 are disposed in a close proximity to each other butare also separated from each other by a small air gap of the order ofseveral micrometers. The air gap may be provided, for example, toaccommodate the height of light extraction features and also to preventoptical contact between the surfaces of the sheets.

Sheet 10 has light extraction features 8 formed in surface 11 and lightextraction features 9 formed in surface 12. Sheet 20 has lightextraction features 91 formed in a broad-area surface 21 and lightextraction features 92 formed in an opposing broad-area surface 22.Sheet 30 has light extraction features 93 formed in a broad-area surface31 and light extraction features 94 formed in an opposing broad-areasurface 32.

According to one embodiment, all of light extraction features 8, 9, 91,92, 93 and 94 are formed by microdots of a UV-curable ink, such aswhite-color ink, or fluorescent ink. The light guiding sheets arepatterned such that, when all three sheets are pressed against eachother, the extraction features formed in one sheet can touch therespective surface of an adjacent sheet. For example, light extractionfeatures 9 can touch surface 21 and light extraction features 91 cantouch surface 12. In this case, the air gap between sheets 10 and 20 maybe primarily defined by the height (or thickness) of light extractionfeatures 9, 91, 92 and/or 93. As shown in FIG. 15, at least some lightextraction features 9 may be disposed in spaces between light extractionfeatures 91 and at least some light extraction features 92 may bedisposed in spaces between light extraction features 93.

Three different arrays of LEDs are provided to independently illuminatelight guiding sheets 10, 20 and 30. Sheet 10 is illuminated by an arrayof LEDs 2 optically coupled to light input edge surface 13, sheet 20 isilluminated by an array of LEDs 2′ optically coupled to a light inputedge surface 96, and sheet 30 is illuminated by an array of LEDs 2″optically coupled to a light input edge surface 97.

According to one embodiment, LEDs 2, 2′ and 2″ may be configured to emitlight in different colors (e.g., LEDs 2 can be red, LEDs 2′ can begreen, and LEDs 2″ can be blue) such that the respective patterns ofsheets 10, 20 and 30 can emit light in the respective colors. Lightguide illumination system 900 of FIG. 15 can be configured independentlyemit light in three different colors simultaneously or in a succession.For example, according to one embodiment, light extraction features 8and 9 may cumulatively form a first pattern configured to display afirst image when illuminated, light extraction features 91 and 92 maycumulatively form a different second pattern configured to display adifferent second image when illuminated, and light extraction features93 and 94 may cumulatively form a different third pattern configured todisplay a different third image when illuminated. LEDs 2, 2′ and/or 2″may be independently controlled and selectively turned on and off toilluminate and display the first, second and/or third pattern or image,respectively. The relative intensity of light emitted from therespective patterns may be controlled by selectively controlling thebrightness of the arrays of LEDs 2, 2′ and/or 2″ (e.g., by individualdimming).

According to one embodiment, light guide illumination system 900 of FIG.15 may be configured to emit light from both outermost surfaces 11 and32 and to be viewable from both sides. According to one embodiment, areflective sheet or surface can be provided on either side (e.g., atsurface 11) to limit light emission to one side only (e.g., surface 32).According to one embodiment, light extraction features 8, 9, 91, 92, 93and/or 94 may have different optical properties (e.g., differentcolors). Accordingly, when illuminated by LEDs 2, 2′ and/or 2″, system900 may be configured to display two, three or more different patternsin different colors or intensity.

According to one embodiment, a single LED source (e.g., LED 2) may beused to illuminate all of the three light guiding sheets 10, 20 and 30.In this case, the LED source should preferably have a light emittingaperture that is equal to or slightly less than the combined thicknessof light guiding sheets 10, 20 and 30 but greater than a combinedthickness of any two of the sheets. According to one embodiment, the LEDsource may be configured to emit light in a single narrow color range(e.g., in blue color) and the light extraction patterns of differentlight guiding sheets 10, 20 and 30 may be configured to extract lightwith different optical effects, e.g., convert light to different colors.

For example, referring to FIG. 15, light extraction features 8 and/or 9may be configured to scatter a blue light without conversion, lightextraction features 91 and/or 92 may include a first fluorescentmaterial configured to convert the blue light into a first color, andlight extraction features 93 and/or 94 may include a second fluorescentmaterial configured to convert the blue light into a second color whichis different than the first color. Accordingly, wide-area light guideillumination system of FIG. 15 may be configured to display differentilluminated patters in different colors from the same wide area evenwhen single-color LED light sources are used.

According to an alternative embodiment, the LED source illuminating allthree light emitting sheets 10, 20 and 30 may be configured to emit agenerally white light and the emission in different colors may beprovided by incorporating various color pigments into the respectivelight extraction features of different sheets. For example, lightextraction features 8 and/or 9 may include a blue or cyan pigment, lightextraction features 91 and/or 92 may include a red or magenta pigment,and light extraction features 93 and/or 94 may include a green or yellowpigment.

According to one embodiment, sheets 10, 20 and 30 may be positionedslightly father apart and optical sheets 71 may be inserted in thespaces between the spaced-apart sheets (FIG. 16). According to oneembodiment, each optical sheet 71 is a transmissive light diffusingsheet. The transmissive light diffusing sheets may be configured, forexample, to mask individual light extraction features or blur theoutlines or optical irregularities of the light extraction patterns.According to one embodiment, additional sheets 71 may also be providedon the sides of surfaces 11 and 32 and configured to diffuse lightemitted from those surfaces. Alternatively, or in addition to that, areflective surface may be provided on either side to recycle lightemitted from the respective surface (11 or 32) and cause emittingsubstantially all of the light extracted from sheets 10, 20 and 30through the opposite surface (32 or 11, respectively).

FIG. 17 schematically illustrates an embodiment of light guide 800including several different types of light extraction features formed inboth opposing broad-area surface 11 and 12. Light extraction features402 exemplify round dome-shaped, fully cured microdots of a UV inkformed on surfaces 11 and 12 of light guiding sheet 10. Light extractionfeatures 404 exemplify flat-top microdots of a UV ink. Each flat-topmicrodot may have a rounded trapezoidal shape having a constant ornear-constant thickness for at least 50%, 60%, 70%, 80% or 90% of itsarea. Light extraction features 406 exemplify small spherically shapedbumps, protrusions or microlenses formed in surfaces 11 and 12. Lightextraction features 408 exemplify rounded dimples formed in surfaces 11and 12. Light extraction features 410 exemplify conical dimples ortriangular prismatic grooves formed in surfaces 11 and 12. Lightextraction features 412 exemplify conical or triangular prismaticprotrusions formed in surfaces 11 and 12. Light extraction features 414exemplify high-aspect-ratio cavities of grooves formed in surfaces 11and 12. By way of example and not limitation, the cavities, dimples orgrooves may be formed by any of the following methods: molding (e.g.,compression or injection molding), embossing (e.g., hot embossing),etching (e.g., chemical or ion bombardment), or laser ablation (e.g.,using a CO₂ laser, especially when sheet 10 is made from an acrylicmaterial).

It is noted that FIG. 17, illustrating a cross-section of light guide800, also exemplifies highly elongated (linear) geometricalconfigurations of light extraction features 402, 404, 406, 408, 410,412, and 414, which have a longitudinal axis perpendicular to the YZplane (or parallel to the X axis). The longitudinal lengths of the lightextraction features 402, 404, 406, 408, 410, 412, and 414 (in a linearconfiguration), as measured along the X axis, may vary in a broad range.According to one embodiment, the lengths of the respective lightextraction features may be at least two or three times of their widthmeasured along the Y axis but much less than the respective dimension ofsheet 10, as measured along the X axis. According to one embodiment, atleast some of linear light extraction features 402, 404, 406, 408, 410,412, and 414 may have lengths that approximates the respective dimensionof sheet 10, as measured along the X axis. According to one embodiment,linear light extraction features 402, 404, 406, 408, 410, 412, and 414may be oriented along a perpendicular direction, e.g., parallel to the Yaxis. According to one embodiment, linear light extraction features 402,404, 406, 408, 410, 412, and 414 may be oriented at an oblique anglewith respect to the X axis and/or Y axis.

It should be understood that the above teachings in reference to lightextraction features 402, 404, 406, 408, 410, 412, and 414 may beapplied, without limitations, to configuring any of light extractionfeatures 8, 9, 91, 92, 93, and 94 described in the precedingembodiments. Furthermore, the different types or configurations of lightextraction features described above can be combined within the samelight guiding sheets (e.g., sheets 10, 20 and/or 30), for example, toachieve different visual effects (e.g., displaying different illuminatedpatterns and/or colors, statically or in succession) or to obtaindifferent predefined luminance distributions. For example, surface 11 oflight guiding sheet 10 may have micro-dots of white UV-curable inkhaving one composition (e.g., a first volumetric density of lightscattering particles within a transparent binder), micro-dots of whiteUV-curable ink having a different composition (e.g., a second volumetricdensity of light scattering particles within the transparent binder),and micro-cavities, dimples or micro-bumps distributed across thesurface according to a two-dimensional pattern. According to oneembodiment, light extraction features of one type or composition may beformed in surface 11 and light extraction features of a different typeor composition may be formed in opposite surface 12.

FIG. 18 through FIG. 21 schematically show polar luminous intensitydistribution graphs calculated using optical raytracing for differentconfigurations and combinations of light extraction features formed inbroad-area surfaces of a thin planar light guide illuminated from twoopposite edges by arrays of LED sources, e.g., as illustrated by theexample of LEDs 2 optically coupled to edge surfaces 13 and 14 inFIG. 1. The sampling plane selected for each graph is perpendicular tothe prevalent plane of the planar light guide and to both light inputedges. In other words, the sampling plane illustratively corresponds toa plane parallel to the YZ plane in FIG. 1 when light input through edgesurfaces 13 and 14 only. Assuming a horizontal orientation of the planarlight guide, the upper half of each graph corresponds to an upwardemission from the top surface of the light guide and the lower half ofeach graph corresponds to a downward emission from the bottom surface ofthe light guide.

By way of example and not limitation, the optical configurations ofwide-area light guide illuminations systems which angular emissiondistributions are illustrated in FIG. 18 through FIG. 20 may beadvantageously selected for direct/indirect illumination (e.g.,incorporated into a suspended lighting fixture) while the configurationcorresponding to FIG. 21 may be advantageously selected for direct-onlyillumination (e.g., incorporated into a recessed or surface-mountdownlight troffer).

FIG. 18 shows a symmetrical luminous intensity distribution in whichlight output from opposite top and bottom broad-area surfaces of thelight guide is near identical in terms of both the angular dependenceand the total energy of emitted light. The angular distribution patternsfor each side (e.g., top and bottom) of the light guide closelyapproximate that of a Lambertian (cosine) emission having an equivalentoverall light output.

FIG. 19 is a calculated polar luminous intensity distribution graph foran alternative exemplary configuration of light extraction featureswhich produce a substantially Lambertian emission from both the top andbottom surfaces with the overall light output from the bottom surfacebeing significantly greater than the overall light output from the topsurface.

FIG. 20 is a calculated polar luminous intensity distribution graph fora further alternative exemplary configuration of light extractionfeatures that produces two different angular emission distributions of a“batwing” type. The top surface produces a “batwing” angular emissionwith a wide throw and a small on-axis “bulge”. The bottom surfaceproduces a “batwing” angular emission with a much narrower throw andwith a reduced on-axis intensity.

FIG. 21 is a calculated polar luminous intensity distribution graph fora yet further alternative exemplary configuration of light extractionfeatures and for the case where a semi-specular reflective sheet isplaced on top of the top surface of the edge-lit light guide such thatthe light is emitted in a downward direction only. The resulting angularintensity distribution is directional with the intensity being nearconstant within a range of angles between about −50° to about +50° froma surface normal.

EXAMPLE 2

A wide-area light guide illumination system was made using an edge-litplanar acrylic light guide having a thickness of about 1.5 mm and majordimensions of about 600 mm by 600 mm. The light guide was patterned onone side using microdots of a UV-cured white ink. The light guide wasilluminated from two opposing edges using two strips of white SMD LEDspositioned in a close proximity to the respective edge surfaces. Thetotal light output of the two bare LED strips (without the light guide)was measured at 4,500 lumens.

The dimensions of the light emitting aperture of each LED were about 1.2mm by 1.2 mm. The light extraction pattern was produced by a randomizedtwo-dimensional array of microdots deposited to the light guide surfaceusing a commercial flatbed UV printing machine (a UV printer). Thedensity of the pattern was made gradually increasing from the lightinput edges towards the center of the light guide. Each light extractionfeature was represented by an individual microdot produced by a singledroplet of the UV ink. Each microdot had a size of approximately 100-130micrometers and a maximum thickness of about 6 micrometers in thecenter. The volume of each microdot was about 40,000 cubic micrometers.No light diffusing or reflective sheets were included into the wide-arealight guide illumination system of this example.

The luminous intensity distributions of the emission from the top andbottom surfaces were measured using a Type C Goniophotometer System. Thelight guide was oriented horizontally with the surface patterned withlight extraction features facing up. The results of the goniophotometricmeasurements are summarized in the annotated polar luminous intensitydistribution graph shown in FIG. 22. The intensity units on the graphare candelas.

Referring to FIG. 22, a curve 801 represents a measured angulardependence of the luminous intensity of the emission from the topsurface of the light guide in a first vertical plane (through horizontalangles 0°-180°). A curve 802 represents a measured angular dependence ofthe luminous intensity of the emission from the top surface in a second(orthogonal) vertical plane (through horizontal angles 90°-270°). Acurve 803 represents a calculated reference angular dependence of theluminous intensity for a Lambertian emitter of a similar total lightoutput compared to the top surface.

Referring further to FIG. 22, a curve 804 represents a measured angulardependence of the luminous intensity of the emission from the bottomsurface of the light guide in a first vertical plane (through horizontalangles 0°-180°). A curve 802 represents a measured angular dependence ofthe luminous intensity of the emission from the bottom surface in asecond (orthogonal) vertical plane (through horizontal angles 90°-270°).A curve 803 represents a calculated reference angular dependence of theluminous intensity for a Lambertian emitter of a similar total lightoutput compared to the bottom surface.

As it can be seen from the graph, both the top and bottom surfacesproduced near-Lambertian angular intensity distributions with thedeviations from the “ideal” Labmertian being about 10% or less for mostobservation angles. The total light output from the patterned topsurface was about 10% greater than that from the non-patterned bottomsurface.

EXAMPLE 3

The wide-area light guide illumination system described the Example 2was modified by adding an opaque diffuse reflector to the top surface ofthe planar light guide and measured using the same procedure. Themeasurement results are summarized in the annotated polar luminousintensity distribution graph shown in FIG. 23. Referring to FIG. 23, thetotal light output from the bottom surface approximately doubledcompared to the Example 2. Furthermore, the measured emission in bothorthogonal planes became even more closely resembling the “ideal”Lambertian emission normalized to the same total light output. Thedeviation between the measured intensity and the calculated intensitybased on the Labmertian law constituted 5% or less for most measuredangles. Due to the high opacity of the top reflector (˜100%), virtuallyno light was emitted from the top surface of the device.

EXAMPLE 4

FIG. 24 shows an annotated photograph of an ordered light extractionpattern of semi-opaque microdots printed on a broad-area surface(exemplifying surface 11) of a planar acrylic sheet (exemplifying lightguiding sheet 10). Each of the printed microdots forms a discrete lightextraction feature on the surface (exemplifying individual lightextraction feature 8). The light extraction pattern was printed using asemi-opaque UV-curable white ink containing TiO₂ particles inconcentrations from 5% to 15% by weight. The printed pattern had a fixedpitch in both X and Y directions and included microdots of differentsizes. At least some of the larger light extraction features 8 wereformed by individual microdrops each having a volume of around 40,000cubic micrometers.

EXAMPLE 5

FIG. 25 shows an annotated photograph exemplifying a random pattern oflight extraction features 8 printed using the same type of ink andprinting hardware as in the Example 4. The resulting printed pattern ofFIG. 25 included a random mix of different shapes (including regular,quasi-regular, round, elongated, irregular and highly irregular shapes),orientation and sizes.

EXAMPLE 6

FIG. 26 shows an annotated photograph of an ordered pattern of opticallyclear microdots (transparent light extraction features 8) printed on asurface of light guiding sheet 10 using a transparent UV-curable ink.The microdots in this print had elongated shapes generally alignedparallel to the X axis.

EXAMPLE 7

Similarly, FIG. 27 shows an annotated photograph of optically clearmicrodots (transparent light extraction features 8) that were printedusing the same type of ink as in the Example 6 but also using a lowerdensity of the pattern (greater spacing distances SPD between adjacentmicrodots).

EXAMPLE 8

FIG. 28 shows an annotated photograph of individual light extractionfeature 8 formed by a microdot of semi-opaque, UV-curable white ink. Themicrodot had a shallow spherical shape with a substantially round,regular outline. The diameter of the shallow dome-shaped microdot wasabout 130 micrometers and the maximum thickness at the center was about8 micrometers.

EXAMPLE 9

FIG. 29 illustrates an individual printed microdot (light extractionfeature 8) which is similar in size and composition to that of theExample 8, except that is has an irregular, elongated shape withsomewhat fuzzy borders at the longitudinal ends of the elongated shape.

EXAMPLE 10

A planar acrylic light guide was patterned for light extraction using atwo-dimensional randomized pattern of microdots formed by a white-colorUV-curable ink including light-scattering nanoparticles suspended in aclear binder material. The microdots were deposited to one of thebroad-area surfaces of the light guide using a different commercial UVprinting machine and using a different printing regime compared to theprevious Examples. The overall light extraction pattern included areasof different distribution density of the microdots (sub-patterns). Aseries of photographs of different sampling areas of the patternedsurface corresponding to different distribution densities of themicrodots was taken using a microscope camera. The resultingphotographs, enumerated a) through g), are shown in FIG. 30. The fieldof view of each photograph is approximately 3.3 mm by 2.5 mm.

Photograph a) shows relatively sparsely populated microdots(representing individual light extraction features 8) that correspond toan area of surface 11 that is relatively close to a light input edge(not shown). The microdot patterns within same-sized sampling areasgradually become denser with the increase of the distance from the lightinput edge, as shown in photographs b) through d), but still withoutsignificant overlaps of individual microdots.

Photograph e) illustrates a sampling area that is even further away fromlight input edge and has an even greater density of the microdots (withlower spacing distances SPD) compared to the sampling areas depicted onphotographs a) through d). As it can be seen from photograph e), atleast some of the adjacent microdots partially overlap one another.

Photograph f) illustrates a sampling area having an even greaterdistribution density of the microdots. There is also a substantialamount of overlap between adjacent microdots (having less-than-zeroseparation distances SED), including groups of 2, 3 or more overlappingmicrodots. Photograph g) depicts a sampling area characterized by yeteven greater density of the microdots which overlap in large groups andcreate a continuous, randomly textured three-dimensional lightextraction surface with occasional random voids 155. Voids 155 weresubstantially free from the light scattering material or contained asubstantially reduced amount of the light scattering material comparedto the adjacent surface textures produced by the fully loaded lightscattering ink material.

FIG. 31 shows a composite three-dimensional image of an individualprinted microdot (exemplifying individual light extraction feature 8)which shape and dimensions were typical for at least some portions ofthe light extraction pattern produced in this Example. The image wasobtained using a true-color 3D surface profiler/microscope from ZetaInstruments (model Zeta-200). FIG. 32 shows a cross-sectional surfaceprofile (a curve 500) of the printed microdot taken along a cuttingplane 99 passing through a mid-section of the microdot.

As can be seen from FIG. 31 and FIG. 32, the exemplary printed microdothad a generally round shape with a “flat-top” (truncated cone) 3Dgeometry, a narrow (<5 micrometers) outer rim with a very low (<0.2-0.3microns) thickness, sloped side walls (visualized as walls 502 of curve500), a diameter between 70 and 80 micrometers, and a near-constantthickness between 1.2 and 1.5 micrometers in the area between the slopedside walls. The volume of the exemplary printed microdot (in the fullycured, solid state) was measured at about 4,000 cubic micrometers.

As demonstrated by surface profile curve 500, the surface of theexemplary microdot had random surface irregularities 504 on a scale ofthe order of 0.1 micrometer. The random surface irregularities can alsobe seen in the form of a visible matte-finish surface texture on themicrophotograph of FIG. 31. According to an aspect, the exemplaryprinted microdot represented a thin, semi-opaque layer of a lightscattering material (a white ink) having a near-constant thickness ofthe order of one micrometer and occupying a round area having a diameterof less than 80 micrometers.

End of Example 10

Further details of operation of illumination systems shown in thedrawing figures as well as their possible variations will be apparentfrom the foregoing description of preferred embodiments. Although thedescription above contains many details, these should not be construedas limiting the scope of the invention but as merely providingillustrations of some of the presently preferred embodiments of thisinvention. Therefore, it will be appreciated that the scope of thepresent invention fully encompasses other embodiments which may becomeobvious to those skilled in the art, and that reference to an element inthe singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

What is claimed is:
 1. A lighting fixture with segmented emission,comprising: a sheet of an optically transmissive material having a firstbroad-area surface configured for light output, an opposing secondbroad-area surface extending parallel to the first broad-area surfaceand configured for light output, a first edge configured for lightinput, and an opposing second edge; a first plurality of compactsolid-state light sources optically coupled to the first edge; aplurality of patterned light extraction areas distributed over an areaof the sheet of the optically transmissive material and separated fromone another and from the first edge by separation areas, each of theplurality of patterned light extraction areas comprising atwo-dimensional pattern of light extraction features formed in or on thefirst or second broad-area surface; and a housing encasing the firstplurality of compact solid-state light sources and the first edge,wherein a width of at least one of the separation areas is less than alength or width dimension of at least one of the plurality of patternedlight extraction areas and at least 10 times greater than a prevalentspacing distance between the light extraction features within the atleast one of the plurality of patterned light extraction areas, andwherein an areal density of the light extraction features at a firstedge of a first one of the plurality of patterned light extraction areasis less than an areal density of the light extraction features at acentral area of the first one of the plurality of patterned lightextraction areas.
 2. A lighting fixture with segmented emission asrecited in claim 1, comprising a second plurality of compact solid-statelight sources optically coupled to the second edge.
 3. A lightingfixture with segmented emission as recited in claim 1, comprising asecond plurality of compact solid-state light sources optically coupledto the second edge, wherein an areal density of the light extractionfeatures at an edge of a second one of the plurality of patterned lightextraction areas is less than an areal density of the light extractionfeatures at a central area of the second one of the plurality ofpatterned light extraction areas.
 4. A lighting fixture with segmentedemission as recited in claim 1, wherein an areal density of the lightextraction features at an edge of a second one of the plurality ofpatterned light extraction areas is less than an areal density of thelight extraction features at a central area of the second one of theplurality of patterned light extraction areas, and wherein an arealdensity of the light extraction features at an edge of a third one ofthe plurality of patterned light extraction areas is less than an arealdensity of the light extraction features at a central area of the thirdone of the plurality of patterned light extraction areas.
 5. A lightingfixture with segmented emission as recited in claim 1, wherein theseparation areas collectively form a substantially transparentcontiguous region surrounding at least one of the plurality of patternedlight extraction areas.
 6. A lighting fixture with segmented emission asrecited in claim 1, wherein the separation areas collectively form asubstantially transparent contiguous region surrounding at least severalones of the plurality of patterned light extraction areas.
 7. A lightingfixture with segmented emission as recited in claim 1, wherein each ofthe plurality of patterned light extraction areas has a generallyrectangular shape, and wherein an areal density of the light extractionfeatures at a second edge of the first one of the plurality of patternedlight extraction areas is less than the areal density of the lightextraction features at the central area.
 8. A lighting fixture withsegmented emission as recited in claim 1, comprising a second pluralityof compact solid-state light sources optically coupled to the secondedge, wherein an areal density of the light extraction features at asecond edge of the first one of the plurality of patterned lightextraction areas is less than the areal density of the light extractionfeatures at the central area.
 9. A lighting fixture with segmentedemission as recited in claim 1, wherein an areal density of the lightextraction features at a second edge of the first one of the pluralityof patterned light extraction areas is less than the areal density ofthe light extraction features at the central area, and wherein the firstand second edges of the first one of the plurality of patterned lightextraction areas are parallel to the first edge of the sheet of anoptically transmissive material.
 10. A lighting fixture with segmentedemission as recited in claim 1, wherein the separation areascollectively form a substantially transparent contiguous region spanningsubstantially all exposed surface area of the sheet of an opticallytransmissive material and encompassing the plurality of patterned lightextraction areas.
 11. A lighting fixture with segmented emission asrecited in claim 1, wherein at least one of the separation areascomprises a two-dimensional pattern of light extraction features whichareal density is much less than an average areal density of an adjacentone of the plurality of patterned light extraction areas.
 12. A lightingfixture with segmented emission as recited in claim 1, wherein at leastsome of the light extraction features are discrete surface structureseach comprising a layer of a light scattering material having a variablethickness characterized by a curved cross-sectional profile, and whereina ratio between a longest dimension of the least one of the lightextraction features and a maximum thickness of the layer of theoptically transmissive material is greater than
 5. 13. A lightingfixture with segmented emission as recited in claim 1, comprising asecond plurality of compact solid-state light sources optically coupledto the second edge, wherein the housing is further encasing the secondplurality of compact solid-state light sources and the second edge. 14.A lighting fixture with segmented emission as recited in claim 1,wherein the width of at least one of the separation areas is at least 50times greater than the prevalent spacing distance.
 15. A lightingfixture with segmented emission as recited in claim 1, wherein the sheetof an optically transmissive material is configured to be retained in acurved shape.
 16. A lighting fixture with segmented emission as recitedin claim 1, wherein at least one of the light extraction featurescomprises a layer of an optically transmissive material having avariable thickness characterized by a curved cross-sectional profile andhas a size between 10 micrometers and 200 micrometers along a longestdimension and a total volume between 1,000 cubic micrometers to 100,000cubic micrometers, and wherein a ratio between the longest dimension anda maximum thickness of the layer of the optically transmissive materialis greater than
 5. 17. A lighting fixture with segmented emission asrecited in claim 1, wherein at least some of the light extractionfeatures are disposed in contact or overlap with one another.
 18. Alighting fixture with segmented emission as recited in claim 1, whereinthe first and second broad-area surfaces are configured to outputapproximately equal amounts of light from the sheet of the opticallytransmissive material.
 19. A lighting fixture with segmented emission,comprising: a sheet of an optically transmissive material having a firstbroad-area surface configured for light output, an opposing secondbroad-area surface extending parallel to the first broad-area surfaceand configured for light output, a first edge configured for lightinput, and an opposing second edge; a first plurality of compactsolid-state light sources optically coupled to the first edge; aplurality of patterned light extraction areas distributed over an areaof the sheet of the optically transmissive material and separated fromone another and from the first edge by separation areas, each of theplurality of patterned light extraction areas comprising atwo-dimensional pattern of light extraction features formed in or on thefirst or second broad-area surface; and a housing covering the firstplurality of compact solid-state light sources and portions of the sheetof the optically transmissive material adjacent to the first and secondedges, wherein a width of at least one of the separation areas is lessthan a length or width dimension of at least one of the plurality ofpatterned light extraction areas and at least 10 times greater than aprevalent spacing distance between the light extraction features withinthe at least one of the plurality of patterned light extraction areas,and wherein an areal density of the light extraction features at a firstedge of a first one of the plurality of patterned light extraction areasis less than an areal density of the light extraction features at acentral area of the first one of the plurality of patterned lightextraction areas.
 20. A lighting fixture with segmented emission,comprising: a sheet of an optically transmissive material having a firstbroad-area surface configured for light output, an opposing secondbroad-area surface extending parallel to the first broad-area surfaceand configured for light output, a first edge configured for lightinput, and an opposing second edge; a first plurality of compactsolid-state light sources optically coupled to the first edge; aplurality of patterned light extraction areas distributed over an areaof the sheet of the optically transmissive material and separated fromone another and from the first edge by separation areas, at least one ofthe plurality of patterned light extraction areas comprising atwo-dimensional pattern of light extraction features formed in or on thefirst or second broad-area surface; and a housing covering the firstplurality of compact solid-state light sources and portions of the sheetof the optically transmissive material adjacent to the first and secondedges, wherein a width of at least one of the separation areas is lessthan a length or width dimension of the at least one of the plurality ofpatterned light extraction areas and at least 10 times greater than aprevalent spacing distance between the light extraction features withinthe at least one of the plurality of patterned light extraction areas.